HOLZFORSCHUNGMITTEILUNGEN ZUR CHEMIE, PHYSIK, BIOLOGIE UND TECHNOLOGIE DES HOLZES

19. Band — Heft 3 TECHNISCHER VERLAG HERBERT CRAM, BERLIN 30HERAUSGEBER DR. G. STEGMANN, BRAUNSCHWEIG Juni 1965

Aspects of the Aging Process in Cambium and Xylemby H. H. Bosshard

Department of Microtechnological Wood Research, Fedetal Institute of Technology, Zürich

The aging processes in a living tree are of a verycomplex nature; they occur at different time intervalsand in different dimensional spaces. The aging pro-cesses within a Vegetation period are most obvious inthe leaves: the swelling bud develops into young, paleleaves in spring, which grow to füll size and obtain afirmer epidermis. After weeks and months they becomediscoloured after the first early frosts in autumn; theaging process, beginning when the leaves develop,gradually comes to a standstill, necrobiosis sets in andtakes a rapid course. In the same space of time, cam-bium and xylem have also undergone an aging processwhich, however, is a process that will continue after thewinter rest period and after the resumption of Vege-tation activity. Cambium and xylem thus age both withthe rhythm of unit time and the integral passage oftime. — While there is no lower limit to unit time, itcan at most comprise a period of Vegetation; long-termdevelopment is unlimited. In the measurable ränge themost aged trees are over 4,000 years old, äs determinedon Pinus pinaster trunks of a virgin pine forest in thehigher regions of Arizona. However, this limit isentirely accidental and applies only unilaterally to thegroup of ring forming trees of temperate climatic zones.The biological phenomenon of the timelessness oforganisms or organs capable of dividing thus becomesdecisive for the individual age of tree: if the aging pro-

cess is not subject to qualitative modifications äs in theabove example of the leaves, necrobiosis will at bestinvolve certain tissues and not the individual äs such.Individuais in which the aging process occurs only inthe quantitative respect lose their age, they become time-less. Life äs such is timeless; aging and death are fixedpoints in life due to environmental circumstances whichdo not äs such correspond to its basic concept. However,of the more highly organized organisms, only individualtree relics are witnesses to this unbroken vitality.

Aging Processes in Cambium and XylemCambium is defined äs initial tissue which comprises

the actively dividing fusiform- and ray-initials. Thecambial zone comprises the phloem- and xylem-parent-cell tissues and their daughter cells which arenot yet differentiated. The width of the "cambial zoneis variable and depends on the vegetational rhythmwhile the width of the cambium is commonly constantand comprises only one layer of cells. — In the xylem,the individual elements are grouped into tissue unitswhich perform specific functions: tracheid or fibreground-tissue is designed to perform mechanical func-tions, the tracheid- or vessel-system for the conductionof water and the ray and parenchyma tissue for storage.

In Scheme i the aging process in cambium, cambialzone and xylem is divided into processes occuring

TissueScheme i. Aging Process in Cambium and Xylem

Unit time dt Time r

CON

.ortU

l•^*.0gCDU

jyi*

Fusiform initial layer

Ray initial layer

Mother cell tissue

Mechanical tissue

Conducting tissue

Storage tissue

bD

co

l4-1CO4-*'Ört

W).Sw>

COl»

3a

r - · ^— Deceleration of division process

— Deceleration of division process

— Cell differentiation— Formation of secondary and

tertiary walls

— Loss of protoplasm— Complete lignification

— Loss of protoplasm— Complete lignification

— Cells lose ability to divide

— Length growth of Initialsto maximum value

— Growth in number of initial cells

— Growth in number of initial cells— Increase of ray matrix

— Pit closure in longitudinaltracheids

— Closure of vessels by tyloses

— Formation of heartwood substances— Formation of tyloses— Gradual necrobiosis

9 Holzforschung. Bd. 19, Heft 3

66 H. H. Bosshard Holzforschung

within a time unit and such äs continue over severaltime units. The morphologically detectable changeswithin the cambial zone here designated äs the quan-t i ta t ive expression of aging have been describedmainly by I. W. Bailey, M. W. Bannan and H. E.D ads well and collaborators in fundamental studies.Within unit time, they comprise, according to measure-ments taken by M. W. Bannan (i955)> a retardationof divisions in the initial layer. The retardation in thedividing process, however, substantially determinesthe later formation of cambial derivatives: in the cam-bial zone, bipolar length- and surface-growth of thecells not highly differentiated are possible under mor-phogenetic laws; outside this zone, however, the shapeand size are permanently fixed. In the event of rapiddivision in the cambium, daughter cells remain in thecambial zone and thus under the action of the growthpromotion for only a short time; in the other case,where division is slow, there is sufficient time for alasting development of both the cell unit äs such andthe cell wall in particular. This regularity can be foundagain, according to measurements made by I. J. W.Bisset and H. E. Dadswell (1950) in the inter-relationship between fibre length and ring f ormation. —Along with the differentation of cells, the cell-wall con-struction is completed (A. B. Wardrop, 1964). Theconcept that cell-wall formation from the early develop-ment of the primary membrane via the secondary wallto the tertiary lamella is a specific phenomenon of theaging of plasma may be unfamiliar but fits well into theoverall picture of cambium aging without the aid ofmechanistic principles of explanation.

The three physiologically distinguishable tissues ofthe xylem are differently affected by aging within a timeunit. The cells of the storage tissue lose their ability todivide; most retain their protoplast and are capable ofactive metabolism. This has initiated alterations in theactivity of the tissue which are here referred to äs thequali tat ive expression of aging. These qualitativechanges lead to necrobiosis. The storage tissue isaffected thereby only after the passage of several timeunits while the mechanical and water-conducting tissuesare affected äs a rule early after differentiation in thattheir cell units lose the protoplasts and are appro-priated to strengthening and conducting functions äsempty Shells.

Beyond the time unit, cambium and xylem are sub-jected to aging processes which may be regarded äs thesummation of individual effects. In the cambial ränge,they occur äs a quantitative development; in the xylem,however, they are qualitative. This differentiation in-dicated basic differences between the cambium and ilsderivatives: äs the process of qualitative aging underoptimal conditions is absent in the meristem, the latterwill hardly be subject to the laws of necrobiosis.According to measurements taken by I. W. Bailey(1923) this is also seen in the morphology of the cam-bium when the length of the fusiform initials that con-tinously increase in the first period of about one hundredyears later asymptotically approach a parallel with thetime axis. This dependence is applicable only to non-storied cambium. Along with the phylogenetic develop-ment, the formative tissue thus loses part of its morpho-logical dependence on time in that no increase in thelength of the initial cells or only minor changes can be

observed in the storied cambium. — Independently ofchanges in the individual initial cells, their numberincreases in the rhythm of anticlinical divisions. Theorder of anticlinical divisions havbeen determined byM. W. Bannan. His observations reveal that a f usi-form initial in young conifer cambium can divide anti-clinally three or four times per year iand only once inan aged cambium; moreover, the products of divisionshow considerably greater vitality in the young for-mative tissue than in the old in terms of the degree oftheir ability to divide.

The aging process in the xylem over extended periodsinvolves mainly the storage tissue. Very decisive pheno-mena are observed which initiate the transformation ofsapwood into heartwood and end in the necrobiosis ofthe storage tissue. The other tissue units are morpho-logically and physiologically fixed; the changes stilloccuring there are of a passive nature and relate mainlyto the bordered-pit closure in tracheids, the blocking ofvessels by tyloses and the deposition of heartwoodsubstances in the cell walls.

Aging of Plasma and Cell-wall FormationSince the micellar structure of cell walls found in the

polarization microscope by A. Frey (1926) could bemade accessible to direct observation in the electronmicroscope, the structural principle of the lignified cellwall may be regarded äs known. While the discussionon the order of individual cell-wall lamellae is not yetterminated, more and more return to the view proposedby A. Frey-Wyssling in 1935 that the common cen-tral lamella and the primary walls adjacent thereto oneither side (compound middle lamella) are followed bya three-layered secondary wall and a tertiary wall onboth sides. What is fundamental for the structure ofthe cell wall in individual lamellae is the particularorientation of microfibrils: In the primary wall thefibrils are oriented crosswise similar to a real fabric,in the secondary walls, however, in parallel, the prin-cipal direction not infrequently changing from onelamella to the next. The tertiary wall is then depositedby the plasma äs an amorphous terminal lamella againstthe cell lumen. Simply and generally defined, any cellwall having a cross-wise network of fibrils can be re-garded äs primary, while membranes with parallelizedfibril texture are of secondary origin. — Primary,secondary and tertiary cell walls, however, are not onlymorphological units. As they represent stages whichf ollow one another with the passage of time, they becomethe quantitative definition of the aging of the cytoplasmbuilding them. The formation of new cell walls isdependent on cells capable of dividing and thus occursonly within the cambial zone. — Fig. i shows thedeposition of a newly formed ray cell wall and revealsthat this primary membrane is laid down during thetelophase of nuclear division. It is likely that such wallformation occurs spontaneously in a random fashionfollowing the principle of maximum disorder. Theyoung cell wall is then enlarged by the plasma appro-priated to the new cell and transformed into ihe secon-dary and tertiary stage by apposition. — The plasmaunit appropriated to a cell in the formation of the pri- ·mary wall, is in unused young condition and under thespecial action of the nucleus which is capable of di-vision; in contrast the deposition of the secondary cell

Bd. 19 (1965) H. 3 Aspects of the Aging Process in Cambium and Xylem

Fig. i. Ray matrix in the cambial zone of Picea abies (Radialsection). Telophase in dividing cell with new formed cell wall

1135 (Microphotography by B. Aleier)

wall — and particularly of the tertiary cell wall is by anaged plasma under the action of the nucleus which isno longer capable of division. In this context, the diffe-rences between the irregulär texture of the primarywall, the parallel texture of the secondary wall and theamorphous texture of the tertiary wall may be regardedäs the quantitative indication of plasma aging. It is inthe nature of aging to display youth, maturity andsenescence; the stage of maturity is commonly the onerichest in energy while youth and senescence are poorerin energy. In the change from the irregulär texture tothe parallel order and finally to the amorphous stage,the three aging stages can be recognized and at thesame time identified äs the products of different for-mative energy. If the reactivaiing energy of the threecell walls required f or their disintegration is taken äs themeasure for the formative energy required, it canreadily be shown that the energy curve of aging is at amaximum in the mature stage of secondary cell wallformation. — The morphological order can thus bedefined by its energy content äs with the ferromag-netism of thin layers. Spontaneous magnetization occursin 'a ferromagnet below Curie temperature indepen-dently of an exterior magnetizing field äs per the thermo-dynamic theory of molecular fields (E. K n eil er, 1962).This magnetization, however, is homogenous only insmall areas (the W ei s s area) and corresponds to theso-called zero structure in which the magnetizationvectors mutually cancel each other. If an exterior mag-netic field is allowed to act on a ferromagnet, its spon-taneous magnetization will change in accordance withthe field intensity applied: the magnetization vectors ofthe Weis s areas are successively oriented in thedirection of the exterior magnetic field; the irregulärstructure of the zero-phase changes into a parallelstructure which can be made visible by the so-calledBitter bands. The condition of parallelization is richerin energy than that of the zero phase. — This should beregarded merely äs an analogy and not äs a mechanisticattempt to explain the structure of parallel texture insecondary walls.

Trans fo rmat ion of Sapwood into HeartwoodThe sapwood/heartwood relation in living trees has

time and again been the subject of comprehensive stu-dies. The literature relating thereto has been largelycollated by H. E. Dadswell und W. E. Hillis (1962).In their assumptions, these authors largely adhere tothe conventional subdivision into sapwood trees(e. g. Alnus incana), ripewood trees (e. g. Abies alba)and trees with regulär (e. g. Pinus species) or irregulär(e. g. Fagus silvatica) heartwood formation. This ter-minology is contestable because it is based mainly onmacroscopically visible characteristics and takes toolittle account of the cytology of the aging storage tissue.Indeed, many indications argue for the assumptionthat the transformation of sapwood into heartwoodrepresents the final phase of an aging process in a livingtree. The newly formed xylem is subject to quali-tat ive aging in that the mechanical and conductingcells äs a rule lose their protoplasts while the storagecells lose their capacity to divide within a time unit(e. g. a period of Vegetation). In both cases, this ini-tiates necrobiosis which occurs at different rates butfinally leads to the death of cells and tissues. Thesenecrobiotic processes should generally be regarded ästhe final phase of xylem aging and manifest themselvesin the transformation of sapwood into heartwood.It should here be observed that heartwood formationcan only proceed from the storage tissue and that thischange is gradual in its action. Differences can bemeasured, in the first place, in the sapwood porlion,but they also reveal themselves in the quality of heart-wood formaüon. In young trees, the sapwood zonecommonly reaches from the cambium to the pith; inolder trees it usually comprises only a number of peri-pheral rings. Exceptions are found in the species pre-viously äs so-called "sapwood trees" which possesssapwood-type wood also in the vicinity of the pith.Too little is äs yet known of the actual changes in theliving tissue of these types in the vicinity of the pith,but we have good evidence of that the cytological agingof cells (A. Frey-Wyssling and H. H. Bosshard,1959) at advanced tree ages is basically the same äs inheartwood forming woods, with the exception that itbegins later. Apart from these so-called sapwood trees,the transformation into heartwood usually begins re-gularly, but at least three modifications requiring to beobserved: (i) woods havinglightheartwood(cf. ripe-wood trees). They clearly reveal necrobiosis of thestorage cells without building large quantities of pig-mented heartwood substances. Occasionally, however,coloured heartwood substances are found in the in-dividual storage cells so that it must be assumed thattheir precursors are formed in the cambium of thesewoods äs well, but obviously remain unpigmented.(2) Woods with obligatory-coloured heartwood(cf. trees with regulär heartwood formation). In thisgroup, which may also be known äs the oak type,pigmented heartwood substances are invariably form-ed in the storage tissue, and are generally capableof penetrating into the cell walls of all tissue units.(3) Woods with f a c u l t a t i v e coloured heartwood(cf. trees with irregulär heartwood). This group maybe designated äs the ash type. The brown heart-wood of an ash need not occur in all samples andneed not affect the entire heartwood portion. In

9*

68 H. H. Bosshard Holzforschung

addition, the pigmented heartwood substances arecommonly retained äs wall coating or droplike inclu-sions in the cells of the storage tissue. In thiscase, the cell walls are not impregnated either in thestorage tissue or in the rest of the xylem (H. H. Bös s-hard, 1955). — According to H. Erdtman andE. Rennerfei t (1944) precursors of the heartwoodphenols are formed in the cambium and are capable ofdiffusion towards the heartwood border radially fromthe sapwood through the ray cells. In so doing, theyare most probably subjected to oxypolymerization,which causes pigmentation and higher molecularity.It is conceivable that the polymerization process is socontrolled in respect of time with the necrobiotic pro-cesses in the obligatory-coloured heartwood speciesthat the heartwood substances are small enough whenthe plasma-semi-permeability ends to emerge into theadjacent tissues from the storage cells through thecell-wall filters. — In the facultative-coloured heari-wood species the tendency to form coloured heartwoodmay be genetically less fixed so that it requires particu-lar exterior influences to initiate the relative reactions.In this case, too, the pigmenied heartwood substancesare formed in \the storage tissue; very likely, theirsynthesis sets in earlier in the still active sapwood sothat when the cell dies they are of such high mole-cularity that they can no longer penetrate through thecell walls. In this case, the coincidence in respect oftime necrobiotic processes and the synthesis of heart-wood-substances would be decisive for the type ofsapwood/heartwood transformation.

- BEVEUPMBff OFWCLEI

200-

150-

K».

100-

50-

VOLUME - OfVELOPMfNT OF NUCLfl

SPKINSSUMMERAtlTÜMNWINTER

0CAMBIUM

10 20l

30 ;o so —60 YEMSWH

Fig. 2. Surface- and Volume-Development of nuclei in ray cellsof ash (Fraxinus excelsior) in the four seasons (Measurements by

U. Hugentobler)

Necrobiosis of the storage tissue was formerly de-scribed äs the alteration of the nucleus which e. g. inconifers, gradually tends from an ellipse towards cir-cular configuration becomes pycnotic and finally dis-appears. In describing the necrobiotic conditions, thedegree of slenderness of the nucleus was previouslyemployed. At present, other measured values are beingtested for their significance. By way of example, thecalculation of nucleus surfaces (Fig. 2) has proved tobe useful. To begin with, the basis was the assumptionthat the activity of the cell should largely be attributedto surface reactions at the border between nucleus and

plasma. Indeed, results show that the nuclei in sapwoodin the vicinity of the cambium are provided with largersurfaces than in the sapwood adjacent to the heartwood,which is true for all seasons. In aädition, cell surfacesare larger in the active spring and summer months thanin autumn and winter. — On the whole, the concept ofthe transformation from sapwood into'heartwood whichis the final stage of the aging process in the storagetissue, proves to be very useful in the organization ofthese complex problems.

SummaryIn this brief paper only some aspects of aging in

cambium and xylem have been considered. It is im-portant that aging is quantitative in the meristemand qualitative in the xylem. Along with the changein the essential cell property — the capacity to divide —necrobiosis of the storage tissue is initiated in the finalphase of xylem aging, which finally causes the sapwoodto be transformed into heartwood. In addition, aging ofwood within a time unit and in time generally is dis-cussed. Aging of the plasma in the cell results in spe-cific cell-wall structure while the aging of the meristemaccounts for the formation of rings in springwoodand summerwood. — The growth laws found byK. Sani o (1872) and the differences later establishedbetween juvenile and adult wood (B. J. Rendle, 1958),however, relate to the aging process in the cambium,which extends through decades and which can bemeasured by the continuous increase in the length ofthe fusiform Initials. In this respect, it is particularlysignificant that the highly developed storied cambiumassumes an exceptional position where the morpholo-gical dimension of the fusiform cells is already found inthe juvenile condition.Aspekte der Alterung von Kambium und Xylem

ZusammenfassungIn der Kürze des vorliegenden Beitrages sind im

wesentlichen nur einige Aspekte der Alterung vonKambium und Xylem genannt worden. Wichtig ist derHinweis, daß es sich im Meristem um eine quanti-tative Alterung handelt, im Xylem aber um einequalitative. Mit der Veränderung der wesentlichstenZellqualität — der Teilungsfähigkeit — wird in derEndphase der Alterung des Xylems die Nekrobiose desSpeichergewebes eingeleitet, die schließlich zur Um-wandlung des Splintes in das Kernholz führt. Weiterwird die Alterung des Holzes innerhalb einer Zeit-einheit und mit der Zeit im allgemeinen dargestellt.Dabei sind vor allem die Vorgänge im Bildungsgewebemomentan. So bedingt die Alterung des Plasmas in derZelle spezifische Zellwand-Strukturen, die Alterungdes Meristems den eigentlichen Jahrringaufbau inFrüh- und Spätholz. — Die von K. Sani o (1872) ge-fundenen Wachstumsgesetze und die später erhobenenUnterschiede des jugendlichen und des alten Holzes(B. J. Rendle, 1958) gehen indessen auf die sich überJahrzehnte hinziehende Alterung des Kambiums zu-rück, meßbar an der stetigen Längenzunahme derFusiform-Initialen. Von besonderer Bedeutung ist indieser Hinsicht die Ausnahmestellung der hochent-wickelten Stockwerk-Kambien, in denen das morpho-logische Maß der Fusiforminitialen schon im Jugend-stadium gefunden wird. · t

Bd. 19 (1965) H. 3 Zur Morphologie der Tracheiden in Zellstoffen aus skandinavischem Birkenholz 69

ReferencesBailey, J. W. (1923). — Amer. Journ. Bot. 10: 499—-509.Bannan, M. W. (1955). — Can. Journ. of Botany 33:133—138·Bisset, I. J. W., and Dadswell, H. E. (1950). — Australien

Forestry 14/1: 17—28.Bosshard, H. H. (1955). — Schweiz. Zeitschrift für das Forst-

wesen, 9/10/106: 592—612.Dadswell, H. E., and Hillis, W. E. (1962). — in W. E. Hillis:

Wood Extractives, Academic Press, New York, 513 p.Erdtman, H., and Rennerfei t , E. (1944). — Svensk Pappers-

tidn. 3/47: 45—56.

Frey, A. (1926). — Jahrb. wiss. Botanik 65: 195—223.Frey-Wyssling, A. (1936). — Protoplasma 25: 261—300.Frey-Wyssling, A., and Bosshard, H. H. (1959)· — Holz-

forschung 5/13: 129—137.Kneller, E. (1962). — Ferromagnetismus, Springer Verlag,

Berlin, 719 p.Rendle, B. J. (1958). — News Bulletin of the I. A. W. A. 2:

1—5.Sanio, K. (1872). — Jahrb. f. Botanik 8: 401—420.Wardrop, A. B. (1964). — in: M. H. Zimmermann, The

Formation of Wood in Forest Trees, Academic Press,New York 562 p.

Zur Morphologie der Tracheiden in Zellstoffenaus skandinavischem BirkenholzVon Georg Jayme und Friedrich Karl Azzola

Mitteilung aus dem Institut für Cellulosechemie mit Holzforschungsstelle der Technischen Hochschule Darmstadt

EinleitungNach gängiger Meinung setzen sich die europäischen

Laubhölzer aus drei Zellelementen zusammen: denSklerenchymfasern, den Gefäßen und den Markstrahl-zellen. Hinzu kommt noch als viertes Element ein meistnur geringer Anteil an Längsparenchymzellen. Dagegenbleiben die Tracheiden meist unbeachtet. Weder H u b e rund Prütz (i) noch die Holzeigenschaftstafel „Birke"(2) bringen eine Bemerkung über Birken-Tracheiden.Nur das holzanatomische Werk von Greguss (3) er-wähnt stichwortartig die Existenz dieses seltenen Zell-elements im Birkenholz. In einer neueren Arbeitwidmet H üb er (4) den Tracheiden und Fasertracheidender Angiospermen eine ganz allgemein gehaltene, kurzeNotiz.

Aufgrund unserer eingehenden Beschäftigung mitder Morphologie der im Rotbuchenholz vorkommendenTracheiden (5) haben wir während morphologischerUntersuchungen an technischen Birkenzel ls toffenauch deren Tracheiden in den Kreis der zu bearbei-tenden Fragen miteinbezogen.

Unters u chungsmaterialAls Untersuchungsmaterial dienten je ein technisch

hergestellter Birken-Papiersulfit- und -Sulfatzellstoff.Beide Stoffe stammten aus Schweden; sie waren ge-bleicht und getrocknet. Die zu untersuchenden Probenwurden mit Brillantkongoblau 2 RW angefärbt (6).

Licht mikroskopische UntersuchungenAls Laubholztracheiden werden Zellen angesprochen,

die dünnwandig, langgestreckt, beiderseits geschlossenund reich getüpfelt sind. Die beiden Pfeile der Abb. iweisen auf eine Zelle hin, die alle genannten Bedin-gungen erfüllt. Sie liegt mitten zwischen Birken-Sklerenchymfasern, wodurch der Unterschied, derzwischen einer Birken Sklerenchymfaser und einerBirkentracheide besteht, deutlich hervortritt. DieBirkentracheide der Abb. i ist 1,15 mm lang und0,06 mm breit. Diese Zelle ist charakteristisch fürBirkentracheiden, woraus man ableiten darf, daß dieBirkentracheiden im Durchschnitt etwas kürzer, dafüraber etwas breiter sind als Birken-Sklerenchymfasern.

Bei stärkerer Vergrößerung tritt der Unterschiedzwischen den Sklerenchymfasern und den Tracheidennoch viel deutlicher hervor. Abb. 2 zeigt einen Aus-schnitt aus Abb. i. Deutlich wird vor allem die fürBirkentracheiden überaus reiche Tüpfelung; tüpfel-arme Bereiche der Zellwand trifft man bei Birken-tracheiden nur selten an. Dagegen sind die Skleren-chymfasern nur spärlich getüpfelt. Während die Tüpfelder Sklerenchymfasern sich mit ihren Spalten denLängsachsen der Zellen anpassen, bilden die Tüpfel derTracheide Abb. 2 mit der Längsachse, einen Winkel,der — wie bei den Buchentracheiden (5) — im Bereichum 45° liegt.

Abb. 3 zeigt als Teilvergrößerung aus Abb. i einEnde der Birkentracheide. Es ist zwar reich getüpfeltjedoch ebenso geschlossen wie die der Sklerenchym-fasern. Auch besitzt es keine leiterförmigen Durch-brechungen, wie sie z. B. bei Buchentracheiden (5) undBirken-Gefäßen (2) auftreten.

Abb. 4 zeigt einen Ausschnitt aus dem mittleren Teileiner anderen Tracheide. Ein Vergleich der beidenTracheiden Abb. 2 und Abb. 4 läßt jedoch trotzgleicher Vergrößerung keine nennenswerten Unter-schiede erkennen, weshalb sich die bei Besprechungder Abb. i bis 3 dargelegten Beobachtungen überBirkentracheiden verallgemeinern lassen sollten.

Bei den Untersuchungen an Buchentracheiden (5)erwiesen sich die Übergangsformen zwischen denTracheiden und Gefäßen einerseits, den Tracheidenund Sklerenchymfasern andererseits als besonders auf-schlußreich. Nach unseren Beobachtungen an beidentechnischen Birkenzellstoffen scheinen im BirkenholzZellen, die den Buchen-Fasertracheiden analog wären,nicht vorzukommen. Dagegen fanden sich in denBirkenzellstoffen ebenfalls Fasern, die man als Gef äß-Tracheiden bezeichnen darf.

Abb. 5 zeigt eine für diese seltene Zellform charak-teristische Birken-Gefäßtracheide. Mit den Tracheidenstimmt sie in ihrer allgemeinen Erscheinung überein:sie ist langgestreckt und läuft gegen ihre beiden Endenhin spitz zu. Allerdings besitzt sie ein etwas größeresLumen. Bei genauerer Betrachtung findet man jedochaußer der reichen Tüpfelung ihrer Oberfläche auchnoch zwei Bereiche mit le i terförmigen Durch-