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Silvae Genetica

ZEITSCHRIFT FDR FORSTGENETIK UND FORSTPFLANZENZDCHTUNG

JOURNAL OF FOREST GENETICS AND FOREST-TREE BREEDING

JOURNAL DE GENETIQUE ET D'AMELIORATION DES ARBRES FORESTIERS

The Austrian x Red Pine Hybrid

By W. B. Critchfield*)

D, Sauerlander's Verlag, Frankfurt a. M,

Silvae Genetica 12, Heft 6,181—212 (Nov.-Dez. 1963)

D 21809 F

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© J. D. Sauerlander's Verlag, Frankfurt a. M., 1963

INHALTSVERZEICHNIS

Geographic and Individual Tree Variation in Some A New Method of Dealing With Results of Provenance

Wood Characters of Teak (Tectona grandis L f.). — Tests. By J. H. Wiersma 200

I. Fibre Length. By S. Kedharnath, V. J. Chacko, Berichte 206

5. K. Gupta and J. D. Matthews 181

The Austrian X Red Pine Hybrid. By W. B. Critchfield 187 Notlce 208

Acute Gamma Irradiation of Quercus Seed. — Its

Effect on Germination and Seedling Growth.

By G. R. Stairs 192

Ausgegeben im Januar 1964

The Austrian x Red Pine Hybrid

By W. B. Critchfield*)

(Received for publication May 22, 1963)

The genetic improvement of red pine (Pinus resinosa

Ait.) presents tree breeders with one of their most difficult

problems. Not only is this valuable species remarkably

uniform, .but until 1955 it resisted all attempts to cross it

with other pines. In that year red pine and Austrian pine

(P. nigra var. austriaca [Hoess] Aschers. & Graebn.) were

successfully crossed at the Institute of Forest Genetics,

Placerville, California. The few hybrid trees from this

cross have been .mentioned and illustrated by Duffield

and Snyder (1958) and Righter (1962).

This cross is the first successful hybridization between

hard pines of the Eastern and Western hemispheres. It is

also one of the few interspecific crosses in Pinus that has

been studied developmental^ (MeWilliam 1959), and we

know at what point the reproductive processes usually

break down. The hybrids may prove to be of economic

value in increasing red pine's resistance to its most serious

pest, the European pine shoot moth (Rhyacionia buoliana

[Schiff]). 'Finally, the hybrids are heterotic, greatly ex

ceeding either parent species in early height growth at

Placerville.

Austrian and red pines are grouped together in all

modern classifications of the pines. Shaw (1914) places

them in the Lariciones, a well defined group that includes

most of the hard pines of the Eastern Hemisphere. The only

Western Hemisphere members of this group are red pine,

a native of northeastern North America, and P. trppicalis

Morelet, a Cuban species.

Austrian pine, in contrast to red pine, has been success

fully crossed with several other species in the Lariciones.

The first authentic pine hybrid, produced by A. F. Blakes-

lee in 1914, was P. nigra X densiflora (Austin 1927). Aus

trian pine has since been crossed with P. sylvestris L. and

several East Asian species. These crosses and unsuccessful

attempts to cross Austrian pine are summarized by Wright

and Gabriel (1958).

The cross between Austrian and red pines has been tried

on a much larger scale than any other cross involving

either species (Johnson and Heimburger 1946; Duffield 1952;

Holst and Heimburger 1955; Wright and Gabriel 1958). Be

tween 1948 and 1955 Wright and his coworkers pollinated

*) Geneticist, Pacific Southwest Forest and Range Experiment

Station, P. O. Box 245, Berkeley 1, California.

more than 300 female strobili in attempts to make this

cross, using both species as female parents. Unsuccessful

attempts at the Institute of Forest Genetics since 1940 total

more than 30 tree X tree combinations and 500 female

strobili.

The developmental stage at which this cross fails has

been described by MeWilliam (1959). Ovules of Austrian

pine pollinated by red pine collapse toward the end of the

first season after pollination and at the beginning of the

second season. Breakdown of the ovule is initiated in the

megaspore and later in the developing female gametophyte.

MeWilliam found red pine pollen growing normally in

about 30 percent of the ovules of Austrian pine. These

ovules are the ilast to break down, and some of them remain

intact until seed coats are formed at about the time of

fertilization, early in the second season of development.

Production and Crossing of the Hybrids

The only successful cross of these two species was made

in May, 1955, between two trees growing in the Eddy

Arboretum at Placerville (table 1). The parents were a 30-

year-old Austrian pine of unknown origin and a 27-year-

old red pine from an 'unspecified Maine source. The cross

yielded 45 sound seed, 42 of which produced seedlings.

During their first growing season, the seedlings were not

readily distinguishable from their Austrian pine half-sibs

growing in adjacent rows (products of the P. nigra X nigra

cross listed in table 1). The following year F. I. Righter

singled out four seedlings as possible hybrids. His selection

was based on their marked superiority in height growth

and on the lighter green of their foliage compared to the

Austrian pine controls.

These putative hybrids were outplanted the following

spring (1959), together with their Austrian pine half-sibs

and unrelated red pines of Wisconsin origin. The out-

planted trees were watered only during their first season

in the field. Most of the Austrian pines had to be replaced

the following year, and nearly all of the red pines have

died since outplanting as a result of severe summer

droughts.

The majority of seedlings from this cross of P. nigra X

P. resinosa have proved to be nonhybrids. Of the eighteen

5-year-olds surviving in 1961, only six were probable hy-

187

Table 1. — Results of several controlled crosses involving P. nigra,

P. resinosa, and their hybrid.

Cross

Polli

nation

year

No.

of

stro—

bili

Per

cent

cones

Total

no.

seeds

per

cone

Per

cent

filled

seed

nigra (N7) X

resinosa (V29)1) 1955 25 60.0 12.3 24.32)

nigra (N7) X nigra

(2-tree mix) 1955 23 100.0 46.3 80.3

nigra (N7) X resinosa

(V29) 1959 146 67.8 22.2 0

nigra (4 trees) X 1955 and

resinosa (V29) 1959 66 43.9 3.7 0

nigra resinosa (11) X

resinosa (V29) 1960 6 100.0 17.7 0

') Numbers in parentheses are Institute of Forest Genetics tree

designations.

2) Includes both hybrids and non-hybrids.

brids — the four trees selected by Righter and two trees

remaining in the nursery. The others were indistinguish

able from the Austrian pine checks. This mixture of hy

brids and nonhybrids may be due to contamination at the

time of pollination, or it may be the result of cone or seed

mixing.

An attempt was made to repeat this cross in 1959, using

the same parent trees. Although the cross was made on a

much larger scale than the 1955 cross, no sound seed was

obtained (table 1).

In both 1955 and 1959 this tree X tree combination

produced a much higher set of hollow seed than other Aus

trian X red pine combinations tried at Placerville. The

high yield of hollow seed from this combination is a prop

erty of the Austrian pine used as female parent, since cros

ses of the red pine parent with several other Austrian pine

female parents yielded few hollow seed (table 1). According

to McWilliam (1959), the ovules of Austrian pine collapse

before the seed coat is formed unless the development of

red pine pollen tubes is normal and vigorous. Since this

particular Austrian pine parent yields many hollow seeeds,

its ovules must provide an unusually favorable environ

ment for the growth of red pine pollen up to the time of

seed coat formation.

The first attempt to backcross one of the hybrids to its

red pine parent was a failure. The backcross yielded no

sound seed and a rather low number of hollow seed (ta

ble 1). The backcross was repeated when the same tree and

one other flowered in 1961, with results not yet known.

Characteristics of the Hybrids

Austrian and red pines differ in many features, although

their general similarity caused some 19th centu.ry botanists

to class red pine as a variety of P. nigra (Shaw 1914, p. 51).

Not all of the differences between the two species could

be used in establishing the identity of the hybrids. Some

gross morphological features such as bud shape and leaf

orientation were too variable to be helpful. The conelet

and cone provide reliable distinctions, but only two hy

brids had flowered by 1961. For this reason we have relied

chiefly on the more stable features of needle structure in

establishing the hybrid identity of the nonflowering trees.

Many characteristics are influenced by tree age, and

comparisons of these features cannot usefully be made be

tween the hybrids and their much older parents. Conse

quently, we also compared the hybrids with related Aus

trian pines and unrelated red pines of the same age.

Conelets and Cones

The conelets of Austrian -and red pines are easy to dis

tinguish (figure 1). The scales of the Austrian pine conelet

are transversely ridged and armed with a small prickle.

Both features are pronounced in the Austrian pine parent.

The smaller conelet of red pine is unarmed, and the ex

posed face of the scale is slightly or not at all ridged. The

hybrid conelet is intermediate in size, and has obscure

prickles and inconspicuous ridges.

Figure 1. — Conelets of Austrian pine parent (left), hybrid (center),

and red pine parent.

The first hybrid to flower matured several cones in 1961.

They could be compared only with cones of the Austrian

pine parent, since the red pine parent rarely produces ma

ture cones and the young Austrian pine controls had not

yet begun to flower. The color of the hybrid cones is much

like the tawny yellow of Austrian pine cones and unlike

the nut brown of red pine cones. In cone size, however, the

hybrid is like red pine, which has cones 4 to 6 cm. long.

The cones of the hybrid, with a mean length of only 5.1 cm.,

are significantly smaller than cones of the Austrian pine

parent (mean length 7.0 cm.).

Time of Flowering and Cone Opening

Red pine flowers in late April at Placerville; Austrian

pine, 2 to 4 weeks later. When past dates of pollen collec

tion and maximum receptivity of female strobili are com

bined, the red pine parent averages 14 days earlier in

flowering time than the Austrian pine parent.

In 1960 the female strobili of the single flowering hybrid

reached maximum receptivity at the end of April, 12 days

after pollen shedding of its red pine parent and 9 days be

fore pollen shedding of the Austrian pine parent. The fol

lowing year, the female strobili of the two flowering hy

brids were at maximum receptivity 4 and 10 days after

pollen shedding of the red pine parent. Although these

dates are not fully comparable, they suggest that the hy

brids fall somewhere between the parents in flowering

time.

Red pine matures its cones several months before Aus

trian pine at Placerville. Cones of red pine regularly

open in the last half of September. Austrian pine cones re

main green throughout the fall, and do not ordinarily ripen

and begin to open until December or January.

In 1961 the cones of the hybrid were intermediate but

erratic in time of opening. By October 10, two of its six

cones had turned brown and the cone scales had begun to

separate. By November 1, when the cones were collected,

three cones had started to open. Not until two months later

did the cones of the Austrian pine parent tree begin to

open. •- '

188

Height Growth

Red pine does not grow nearly as well as Austrian pine

at Placerville. The red pine parent tree, one of the most

vigorous red pines in the Eddy Arboretum, was only 37 feet

tall in 1961 at 34 years. The Austrian pines used in the in

terspecific and check crosses were much taller: 52 to 54 feet

at 35 to 37 years.

The hybrids have grown faster from the start than either

parent species. By the end of their first growing season,

they were nearly twice the height of their Austrian pine

half-sibs and three times the height of the red pine checks

(table 2). They have maintained this superiority since out-

planting. At 5 years the hybrids were more than two and

one half times as tall as their Austrian pine relatives and

almost five times as tall as the two surviving red pines

(table 2).

The superiority of the hybrids in height at 5 years may

•be somewhat exaggerated. Four of the five outplanted

Austrian pines are replacements outplanted a year after

the hybrids. The trees they replaced had suffered severe

rabbit damage during their first winter in the field, while

the taller hybrids in an adjacent row suffered only minor

damage to their lower branches. The extra year the re

placements spent in the nursery may have affected total

height at 5 years, but is unlikely to have had much in

fluence on fifth-year increment. During their fifth growing

season (1961), the height growth of the hybrids was more

than double that of the Austrian pine checks (table 2).

The heterotic height growth of the hybrids may have

produced structural unbalance in some of the trees. In

these cases the rapid development of the crown appears to

have outstripped the ability of the trunk to support the

Table 2. — Early height growth of hybrids and parent species.

Species or hybrid

nigra (N7) X

resinosa (V29)

nigra (N7) X nigra

(2-tree mix)

resinosa (Wisconsin)

Number

of

trees

6.41)

53)

2

Mean ht.

at

6 months

0.28

2)

0.15

0.11

Mean 5th-

year ht.

increment

Feet

1.28

2)

0.51

0.43

Mean

ht.at

5 years

4.85

2)

1.77

1.01

') Six-month height includes all six hybrids; later measurements

include only the four outplanted trees.

!) Highly significant difference.

3) Includes one survivor of first outplanting and four replace

ments.

crown. These trees have developed pronounced leans and

had to be mechanically supported.

Leaves

The needles of Austrian and red pines differ in dimen

sions, texture, and structure. Two anatomical charac

teristics — the amount of hypoderm and the location of the

resin canals — have provided the most conclusive evidence

of hybrid identity, but several other leaf characteristics

have also proved to be useful.

Austrian pine has shorter and wider leaves than red pine

(table 3). In both species these dimensions are highly vari

able and affected by tree age, young trees having shorter

and narrower leaves than older ones (table 3). The leaves

of the hybrids are intermediate in length and width com

pared to Austrian and red pines of the same age. The dif

ferences between the hybrids and their Austrian pine sib

lings, however, are not statistically significant (table 3).

The leaves of Austrian pine are straight and stiff, and do

not break off when they are bent double. Red pine leaves

are flexible and break off cleanly when they are bent

double. The leaves of the hybrids are intermediate in stiff

ness. They often break off when they are bent double, but

the break is ragged compared to that of red pine.

This difference in texture reflects the great difference

between the two species in the abundance of thick-walled

cells within the leaf. The fragile needles of red pine have

one, or occasionally two, layers of relatively thin-walled

hypodermal cells. They have no fibers in the transfusion

tissue surrounding the vascular bundles, 'but the cells of

the endodermis have thickened outer walls (Harlow 1931).

The tougher needles of Austrian pine have a thicker epi

dermis, a much more extensive hypodermis, and a layer of

fibers in the transfusion tissue (figure 2; and Harlow 1931).

The hypodermis is usually three cell layers thick (occa

sionally two or four), and the cells of the inner layers are

very thick-walled (figure 3). The endodermis is uniformly

thin-walled.

The leaves of the hybrids are intermediate in some of

these anatomical features and resemble Austrian pine in

others. Both the epidermis and the hypodermis are inter

mediate, but the hybrids resemble Austrian pine in having

a layer of fibers in the transfusion tissue (figure 2). The

hybrids and the young red pine controls lack the thickened

outer endodermal walls of the red pine parent, suggesting

that this characteristic may be influenced by tree age.

Of these anatomical characteristics, the hypodermis is

the most useful in establishing hybrid identity. In the hy-

Table 3. — Leaf characteristics of hybrids and parent species').

Species or hybrid

nigra (N7)

nigra (N7) X nigra

(2-tree mix)

nigra (N7) X

resinosa (V29)

resinosa

(Wisconsin)

resinosa (V29)

No.

of

trees

1

4

5

2

1

Age

Years

37

5

5

5

34

Mean

length

Cm.

11.9

8.8

2)

10.1

12.1

12.7

Mean

width

Mm.

1.82

1.52

2)

1.40

1.32

1.39

No. resin

canals

per leaf

14.2

4.28

2)

3.78

2.0

5.3

Location of resin canals

Principal

Med.

100

100

83

5

5

Ext.

Accessory

Med.

Percent

0

0

17

95

95

100

98

57

0

73

Ext.

0

2

43

0

27

•) Based on. 10-leaf samples from vigorous primary branches. Observations on midportion

of leaves.

*) No significant difference.

189

dermal layersN

accessory resin^canaL

endodermis

.fibers

principal resin'canal

principal^resin canal

Figure 2. — Leaves of parent species and hybrid in transection. (A) 5-year-old Austrian pine

half-sib (Ni-50). (B) Austrian pine parent. (C) 5-year-old hybrid (NiRe-ll).(D) Red pine parent.

mesophyll

hypodermis

} epidermis

cuticle

Figure 3. — Dermal layers of the leaf in parent species and hybrid. Portion shown lies be

tween lines of stomata near the middle of the abaxial surface. (A) 5-year-old Austrian pine

half-sib (Ni-50). (B) Austrian pine parent. (C) 5-year-old hybrid (NiRe-11). (D) Red pine parent.

brids it usually consists of only two layers of cells. The

hybrid leaves lack the very thick-walled inner hypodermal

cells characteristic of the parent Austrian pine and the

young Austrian pine controls (figure 3).

Austrian and red pines have two principal resin canals

and a variable number of accessory canals in each leaf.

The principal canals are often larger than the accessories

and lie near the lateral margins of the needle (figure 2). The

distinction between the two kinds of resin canals is a

critical one, since the principal canals are relatively stable

in presence and position, whereas the accessory canals

often vary in number and position within a single leaf.

The principal canals of red pine are nearly always ex

ternal. In this position the sheath of cells surrounding the

resin canal is continuous with the hypodermis inside the

flat adaxial face of the needle (figure 2). Occasionally the

principal canals are medial (table 3). Accessory canals,

when they are present, may be either medial or external.

The red pine parent has predominantly medial accessory

canals (table 3).

The principal resin canals of Austrian pine are always

medial, and so are almost all of the accessory canals (ta

ble 3). In a total sample of 145 leaves from 12 trees of

varying ages, 97 percent of the accessory canals were me

dial, 2 percent internal, and less than 1 percent external. In

the Austrian pine parent all resin canals, both principal

and accessory, are medial (table 3).

The hybrids are extremely variable in resin canal loca

tion. Three trees can be distinguished from both parent

species solely iby the location of their principal canals,

which may be either medial or external. Unlike the ex

ternal canals of red pine, however, those of the hybrids are

always adjacent to the hypodermis of the convex abaxial

surface of the needle (figure 2). The external position is

rare in one tree and common in the other two.

Neither parent species ever has principal resin canals in

this location. They are never external in Austrian pine,

while in red pine they are always adjacent to the flat inner

face of the needle, never the rounded outer face. This re

markable feature, which sets off these hybrids from both

parent species, is rather common elsewhere in the Laricio-

nes group. The principal resin canals of P. sylvestris, P.

montana, and several other species characteristically oc

cupy this position (Shaw 1914; Harlow 1931).

Four of the hybrids, including the three above, can also

be distinguished from Austrian pine by the high proportion

of external accessory canals. The highest incidence of ex

ternal accessories observed in an individual Austrian pine

was 2 out of 43 in a 10-leaf sample. In the four hybrids, 50

to 70 percent of the accessories are external, the rest me

dial.

190

The remaining two hybrids are indistinguishable from

Austrian pine in resin canal location. The identification of

these trees, neither of which has flowered, rests on their

leaf dimensions and hypodermal characteristics.

Discussion

The few hybrid trees described here are the sole product

of many attempts to cross Austrian and red pines at Placer

ville and elsewhere, making this perhaps the most difficult

cross so far successfully attempted in Pinus. A further in

dication of the incompatibility barrier between the two

species is the failure of our first attempt to backcross one

of the hybrids to red pine. This failure was unexpected,

since most pine hybrids backcross fairly readily with their

parent species even when the parents themselves are dif

ficult to cross.

This failure, and our inability to repeat the cross be

tween Austrian and red pines, focuses attention on the

circumstances of the original cross. Its most noteworthy

feature was the mixed progeny resulting from it, mostly

Austrian pines with a minority of hybrids. We are inclined

to attribute the mixed progeny to contamination with Aus

trian pine pollen at the time of pollination, although the

possibility of later mixing cannot be eliminated.

Deliberate contamination with maternal-parent pollen to

improve the chances of success of a cross is a standard

technique of Michurinist tree breeding as practiced in the

U. S. S. R. (Forest Service 1961). The same technique has

been tried with unknown sucess in an interspecific pine

cross in Sweden (Johnsson 1956).

We think that an accidental admixture of Austrian pine

pollen may have been responsible for our isolated success

in crossing these two species. The structure of the female

gametophyte of Pinus provides a mechanism by which ma

ternal-parent pollen contamination might help in effecting

a difficult cross in this genus. Each gametophyte has sev

eral eggs available for fertilization, .but usually only one

of the resulting zygotes develops into a mature embryo

(Foster and Gifford 1959, p. 409). Austrian pine pollen, if

it were mixed with red pine pollen in amounts so small

that only a fraction of the available eggs could be ferti

lized, would prevent the collapse of the female gameto

phyte observed by MeWilliam (1959). Under these circum

stances the red pine pollen tubes which develop normally

in Austrian pine ovules might be able to continue their

development to the point of fertilization. The postem-

■bryonic vigor of the hybrids makes it unlikely that em

bryonic selection following fertilization would discriminate

heavily against the hybrid embryos.

On the basis of this hypothesis we repeated the cross in

the spring of 1962. The red pine pollen was deliberately

contaminated with small amounts of live Austrian pine

pollen. We also used red pine pollen diluted with large

amounts of dead or homogenized Austrian pine pollen, in

hopes that one of these combinations will enable us to re

produce this hybrid.

These hybrids, if they can eventually be backcrossed to

red pine, will provide the first opportunity to improve this

species by introducing genes from other species. This

source of genetic material is important to red pine because

of its apparent genetic uniformity (Buckman and Buchman

1962; Fowler 1962) and its serious insect pests. One of these,

the European pine tip moth (Rhyacionia buoliana [Schiff]),

is so damaging that it limits the use of red pine in parts of

the northeastern United States and eastern Canada (Holst

and Heimburger 1955; Wright and Gabriel 1958).

Austrian pine is less susceptible than red pine to shoot

moth damage (Holst and Heimburger 1955; Miller and

Heikkenen 1959). Backcrosses of the hybrid, or backcross

derivatives, might reduce the susceptibility of red pine in

two ways: (a) by introducing Austrian pine genes which

lessen susceptibility; and (b) by increasing the growth rate

of red pine, since faster growing red pine is less suscep

tible to tip moth damage (Heikkenen and Miller 1960).

These possibilities will be explored as the hybrids start to

flower more regularly and abundantly.

Summary

In 1955 Pinus nigra (Austrian pine) and P. resinosa (red

pine) were successfully crossed at the Institute of Forest

Genetics, Placerville, California. This cross is the first in

volving Eastern and Western hemisphere hard pines and

the first cross of red pine with any other species. An at

tempt to repeat the cross was unsuccessful. Our isolated

success may have been due to contamination with mater

nal-species pollen, and we are now trying to reproduce it

by using mixtures of pollen of the two parent species. The

six hybrids are intermediate between their parents in most

characteristics, but they greatly exceed either parent in

early height growth. In one feature, the location of the

principal resin canals in the leaves, some of the hybrids

are unlike either parent species but resemble other pines

in the Lariciones group. The hybrids may prove useful in

the transfer of genes between the two parent species, and

particularly in the introduction into red pine of genes af

fecting insect resistance.

Resume

Titre de l'article: L'hybride entre le Pin noir d'Autriche

et Pinus resinosa.

En 1955, on a croise avec succes a l'lnstitut de Genetique

Forestiere de Placerville (Californie) Pinus nigra (Pin noir

d'Autriche) et Pinus resinosa (red pine). Ce croisement est

le premier qui concerne 2 pins du sous-genre Eupinus,

Tun americain, Tautre eurasiatique, et egalement le pre

mier croisement de Pinus resinosa avec n'importe quelle

autre espece. Un essai de repeter ce croisement a abouti

a un echec. Notre succes isole a peut-etre ete du a une

contamination avec du pollen maternel et nous essayons

maintenant de la reproduire avec des melanges de pollen

des 2 especes parentes. Les 6 hybrides sont intermediaires

pour la plupart des caracteres entre les 2 parents, mais ils

depassent nettement Tun et l'autre en ce qui concerne la

croissance initiale en hauteur. Pour un caractere, la situa

tion des principaux canaux resiniferes dans les aiguilles,

certains hybrides ne ressemblent a aucun des parents, mais

ressemblent a d'autres pins du groupe Lariciones. Les hy

brides se reveleront peut-etre utiles pour transferer des

genes d'un des 2 parents a l'autre et en particulier pour

introduire dans le genome de Pinus resinosa des genes de

resistance aux insectes.

Zusammenfassung

Titel der Arbeit: Vber den Bastard Pinus nigra X Pinus

resinosa.

1955 wurden vom Institut fur Forstgenetik in Placer

ville (Californien) Pinus nigra (Schwarzkiefer) und Pinus

resinosa (Rotkiefer) erfolgreich gekreuzt. Diese Kreuzung

war die erste zwischen eurasischen und amerikanischen

191

Kiefern und die erste Artkreuzung mit Pinus resinosa

iiberhaupt. Eine Wiederholung der Herstellung des Bastar-

■des gelang nicht. Unser einmaliger Erfolg konnte durch

Vermischung mit Pollen der Mutterart hervorgerufen wor-

den sein, weshalb wir jetzt seine Reproduktion mit einer

Pollenmischung von beiden Elternarten versuchen. Die

sechs Bastarde sind in ihren meisten Merkmalen interme-

diar, sie iibertreffen aber jeden Elter wesentlich im frii-

hen Hohenwachstum. In einem Merkmal, ahnlich der Lage

der Hauptharzkanale in den Nadeln, unterscheiden sich

einige Bastarde von beiden Elternarten und ahneln ande-

ren Kiefern aus der Gruppe der Lariciones. Diese Bastar

de konnen sich als nutzlich erweisen bei der "Obertragung

von Genen zwischen den zwei Spezies, und besonders fur

die Einbringung von Genen fur Insektenresistenz in die

Pinus resinosa.

Literature Cited

Austin, L.: A new enterprise in forest tree breeding. Jour. For

estry 25: 928—953 (1927). — Buckman, R. E., and Buchman, R. G.: Red

pine plantation with 48 sources of seed shows little variation in

height at 27 years of age. Lake States Forest Exp. Sta. Tech.

Note 616, 2 pp. (1962). — Duffield, J. W.: Relationships and species

hybridization in the genus Pinus. Z. Forstgenetik 1: 93—97 (1952). —

Duffield, J. W., and Snyder, E. B.: Benefits from hybridizing

American forest tree species. Jour. Forestry 56: 809—815, illus.

(1958). — Forest Service: Forestry and forest industry in the

U. S. S. R. Report of a technical study group. U. S. Dept. Agric.

92 pp., illus. (1961). — Foster, A. S., and Gifford, E. M., Jr: Com

parative morphology of vascular plants. San Francisco: W. H.

Freeman & Co. 555 pp., illus.- (1959).-— Fowler, D. P.: Initial studies

indicate Pinus resinosa little affected by selfing. Proc. 9th North

east. Forest Tree Improvement Conference: 3—8. illus. (1962). —

Harlow, W. M.: The identification of the pines of the United

States, native and introduced, by needle structure. New York

State College of Forestry Tech. Bull 32, 21 pp. illus. (1931). —

Heikkenen, H. J., and Miller, W. E.: European pine shoot moth

damage as related to red pine growth. Lake States Forest Expt.

Sta. Paper 83, 12 pp. illus. (1960). — Holst, M., and Heimburger, C:

The breeding of hard pine types resistant to the European pine

shoot moth (Rhyacionia buolinna Schiff). Forestry Chron. 31: 162—

169 (1955). — Johnson, L. P. V., and Heimburger, C: Preliminary

report on interspecific hybridization in forest trees. Canad. Jour.

Res., Sect. C, 24: 308—312 (1946). — Johnsson, H.: (Annual report on

the activities of the Association for Forest Tree Breeding during

1955.) Arsberrat. Foren. Vaxtf. Skogstr. 1955: 3—18. (Swedish) (Plant

Breed. Abstr. 26: 3792) (1956). — McWilliam, J. R.: Interspecific

incompatibility in Pinus. Amer. Jour. Bot. 46: 425—433, illus. (1959).

— Miller, W. E., and Heikkenen, H. J.: The relative susceptibility

of eight pine species to European pine shoot moth attack in Michi

gan. Jour. Forestry 57: 912—914, illus. (1959). — Righter, F. I.: Evi

dence of hybrid vigor in forest trees. In: Tree Growth, ed. T. T.

Kozlowski, N. Y. Ronald Press, 345—355 pp., illus. (1962). — Shaw,

G. R.: The genus Pinus. Pub. Arnold Arboretum 5, 96 pp.. illus.

(1914). — Wright, J. W., and Gabriel, W. J.: Species hybridization

in the hard pines, series Sylvestres. Silvae Genet. 7: 109—115 (1958).

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