An introduction to general biology online

p, pinna ; r.h, portion of main rhizome ; ;r, younger pinna, which is shown en-



larged at B. This pinna is nearly similar to the pinnules of older pinnae.



(Xi.>



AERIAL AND UNDERGROUyD PARTS. 109

below the surface, and brandies widely in various directions.
It may often be followed for a loui^ distance, and in snch casfs.
reveals a surprisingly complicated system of undori^round
branches. At Urst sight, the underground porti(»n of the fern
appears to be the root, but a closer examination shows it t»» be
really the stem or axis of the plant, which ditfers frcjm ordinary
stems chiefly in the fact that it lies horizontallv uiidvr the
ground instead of rising vertically above it. The aerial }K»rtion,
which is often taken for stem and leaf, is reallv leaf onlv. The
true roots are the line fibres which spring in great abundance
from the underground stem. Underground stems more or less
like that of Pteris are not uncommon — occurrini^, for iii>t;iiice,
in the potato, the Solomon' s-seal, the onion, etc. In /'A/'/.v,
and in certain other cases, the underground stem is technically
called the rootstock or rhizome^ and in this plant it constitntes.
the larger and more persistent part of the organism. In the
specimen shown in Fig. 45 the rhizome was about eight feet
long and bore two leaves. It was dug out of sandy soil on tlie
edge of a woodland, and lay from one to six inches below the
surface. It w^as crossed and recrossed in all directions, both
above and below, by the rhizomes of its neighbors, the whole
constituting a coarse network of underground stems loosely till-
ing the upper layer of the soil.

The aerial portion {t\\Q frond or leaf) is likewise divisible
into a number of parts, comprising in the first place the h-af-
stalk or stipe^ and tlie leaf proper or lamina. The latter is suImH-
vided like a feather {pinnately) into a number of lobes {j^'n/hBj
Fig. 44), which vary in form according to the state of de-
velopment of the leaf. In large leaves the two lower i)inna^ are
often larger than the others, so that the leaf appears to consist
of three principal divisions, and is said to be " t^'rfHftr " or tri]»
ly divided (Fig. 44, A). Each pimia is in tnrn pinnately sub-
divided into pinnules (pinnuke) or leaflets (Fig. 44, />), eadi of
which is traversed down the middle by a thickened ridge or
rod, the midrib. The leaflets sometimes liave smo»>th «>ntlines,
but are usually lobed along tlie edges, as in Fig. 44. />. In
this case their form is said to be pinnatifid. Each lohe is like-
wise furnished with a midrib. The stipe enlarges somewliat
just below the surface of the ground, then grows smaller and



110



THE BIOLOGY OF A PLANT.




joins the rliizome. The enlarge-
ment is of considerable interest,
for it occurs at precisely the
point of greatest strain when the
leaf is bent bj the wind or other-
wise, and must serve to strength-
en the stipe.

It will appear from the fol-
lowing description that the plant
body exhibits in some measure
certain general forms of sym-
metry and differentiation which
in a broad sense may be regarded
as analogous to those occurring in
the animal. The rhizome grows
only at one end, and in its struc-
ture suggests the antero-posterior
differentiation of the animal. It
also shows a slight differentiation
between the upper and lower
surfaces, which appears both in
the external form and in the ar-
ran element of the internal lines.
It is furthermore distinctly bilat-
eral, a vertical plane dividing it
into closely similar halves. These
features are, however, far less
prominent in the fern than in
the earthworm, and in plants
they never attain a high degree
of development, while in the
higher animals they are among
the most conspicuous and im-
portant features of the body.
Fig 45— An entire C)f morc general importance in
Sf'^the 'Tllv;s''?s the fern is the repetition of

young and small, • m i. /i i x

and a comparison of Similar parts (brauchcs, roots,

the figure wiUi Fig, , v , ., . i • i

44TN'iiishowsomeof Icaves) along the axis, wnicii

tliG cliff crdicGS Tdg-

tween leares of dif - suffsiests, pei'liaps, a Certain an-

ferentages. ^^ ' "^ -^ '



AXIS AND APPENDAGES. Ill

aloo^y to animal nietamerism, tliouo^li not ii.>iiallv recc-iiizt'd
or designated by the same term. All of these conditions of
differentiation and symmetry are more easily made out l)y an
examination of the aerial portion.

The plant as a whole, may be regarded as cousistin<'" of
an axis (the rhizome and its branches) whifli ]»oars a number
of appendages in the form of roots and lea\'es. The axis fonus
the central body or trunk of the plant, and in it m<jst of its mat-
ter and energy are stored; the appendages are organs for takiu"-
in food, for excretion, for respiration, for repro<luction, etc.

The Underground Stem, or Rhizome, and its Branches. The
rhizome is a hard black, elongated, and branching stem, gener-
ally flattened somewhat in the vertical direction as it lies in the
earth, and expanded slightly on either side to form well-marked
lateral folds — the lateral ridges. Its thickness is seldom more
than half an inch, and usually considerably less. In transverse
section it has the outline shown in Fig. 48, and the marginal
part only is black. The branches repeat in all respects the form
and structure of the main axis. Both the main axis and the
branches end either in conical, pointed, and fleshy structures
about two inches long, or in blunt, yellowish knol)s, ])lainly de-
pressed in the centre. At these ends tlie rhizome grows ; hence
they are called the growing points or apical huds (Figs. 44, 47).
Besides the apical buds the rhizome bears nearly always one
or more dead, decaying tips. These arise in the f(^llowing man-
ner : After attaining a certain length both the rhizome and its
branches gradually die away behind. Death of the hinder part
follows at about the same rate with which growth advances at
the apical buds; so that the total length may not change mate-
rially from year to year. It is ob^'ious that this {)rocess must
result in the gradual and successive detachment of the brandies
from the main axis. Each branch, now become an inde]>end-
ent rhizome, repeats the process; and in this manner a single
original rhizome may give rise to large numbers of distinct
plants, all of which have been at some time in material connec-
tion with an ancestral stock. This process is evidently a kind of
reproduction (though it is not the most important or most obvi-
ous means for the propagation of the plant), and in this way a
large area may be occupied by distinct, though related, plants



112



THE BIOLOGY OF A PLAN I.



whose brandling rhizomes cross and recross, making the subter-
ranean network ah-eady described, p. 109.

Origin of Leaves upon the Rhizome and its Branches. The
young plant of Pteris puts up a number of leaves (7-12) yearly,
but the adult generally develops one only, which grows very
slowly, requiring two years before it unfolds. Towards the end
of the first year it is recognizable only as a mmute knob at the
bottom of a depression near the growing point. At the begin-
ning of the second year it is perhaps an inch high, the stalk




Fig. 46. (After Sachs.)— Developing leaf, etc., of Pteris. A, end of a branch show-
ing the apical bud and the rudiment of a leaf; B, a rudimentary leaf ; C,a
similar leaf in longitudinal section, showing the infolded lamina (?), the attach-
ment to the rhizome, and the prolongation of the tissues of the latter into the
leaf ; D, lamina of a very young leaf ; E, horizontal section through a growing
point which has just forked to form two apical buds. a.h. apical bud; ep, epi-
dermis and underlying sclerotic parenchyma ; f.b, fibro- vascular bundles ; Z,
lamina ; r, root ; s.p, sclerotic prosenchyma ; a:, an adventitious bud at the base
of the leaf.

only having appeared. At the end of the second year the lamina
is developed, and hangs down as shown in Fig. 46, 0. Early in
the spring of the third year it breaks through the ground, and
grows rapidly to the fully-matured state.



LEAVES AND lilllZOME.



113



The leaves usually arise near tlie apical l)U(ls of the main
axis or of the branches. Behind eacli mature leaf remnants of
the leaves of preceding years are often to he found, altfrnatinj;
on tlie sides of the rhizome in reguhir successicjn, and showing
various stages of decay. The iirst of these (whicli is on the
opposite side of the rhizome from the hving leaf) was aUve the
previous year; the next (on the same side with the hving Icafi is
the leaf of the year before that; and s(j on. Fig. 47 sIkhvs an
example of this sort. The leaf of the present year, l\ is fully




Fig. 47. (After Sachs.)— Branch of a rhizome of Pteri.% showing the apical
the stumps of a number of successive leaves (?', P, P, etc.), and a part of
rnizome (rh), r, root.



bud (a.h\
tho main



developed; and the relics of tlie leaves of the ])receding years
are indicated at l\ 1% etc. ; r is the rudiment of next year's leaf.

Internal Structure of the Rhizome. The rhizome is a nearly
solid mass, consisting: of manv different kinds of cells, united
into different tissues, and having a very complicated arrange-
ment. Its study is somewhat difficnlt. Nevertheless the ar-
rangement of the cells is definite and constant, and merits careful
attention, since it has many features which are characteristic <»f
the cellular structure of the stems of higher ])lants. AVe shall
first examine its more obvious anatomv as dis])lave<l in transverse
and longitudinal sections, afterwards making a careful micro-
scopical study of the cells and tissues.

Seen with a hand-lens or tlie naked eye, a transverse section
of the rhizome (Fig. 48) presents a white or yellowish hack-



114



THE BIOLOGY OF A PLANT.



pround bounded bj a black margin (the epidermii) and marked
by various colored or pale spots and bands ; the latter are differ-
ent tissues, or systems of tissue. These different structures are
arranged in three groups or systems of tissue^ which are found




spro



Fig. 48.— Cross-section of the rhizome of Pteris. Lr, lateral ridges; /.p, fundamental
parenchyma; s.p, sclerotic parenchyma; s.pro, sclerotic prosenchyma; /.b, x
flbro-vascular bundles.

among all higher plants in essentially the same form, though
differing widely in the minor details of their arrangement.
These are : —

I. The Fundamental System of Tissues.
II. The Epidermal System.
III. The Fibro-vascular System.

The Fundmnental system consists in Pteris of three tissues :

(ci) fimdainental parenchyma (Fig. 4:8,y.j9), the soft whitish
mass forming the principal substance of the rhizome ;

(5) sclerotic parenchym^a (s.p), the brown hard tissue lying
just below the epidermis, from which it is scarcely distinguish-
able;

(c) sclerotic prosencJiym^a (s.pro), black or reddish dots and
bands of extremely hard tissue, most of which is contained in two
conspicuous bands lying one on either side of a plane joining
the lateral ridges.



TUE GREAT TISSUE-SYSTEMS. 115

Tlie sclerotic pareiiclivnia and the sclerotic i^rosencliviiiu Ijutli
arise through a transforniatioii (hardening, ctc.j of portions of
originally-soft fundamental parenchyma. In most ])lants ahove
the ferns the fundamental system contains neither of these tissues.

The Flhro-vascular system is composed of loiitrituchnal
threads or strands of tissue known as i\nd jihro-vascular hum/les^
and these in one form or another are characteristic of all hiirhcr
plants. They appear here and there in the section (Fig. 48,/'.^)
as indistinct, pale or silvery areas of a roundish, oval, or elon-
gated shape. Closely examined they show an open texture, un-
closing spaces which are sections of empty tubes, or vessels and
fibres, from which the .bundles take their name.

The Epidermal system consists of a single tissue, the epider-
mis^ which covers the outside of the rhizome.

By a simple dissection of the stem with a knife the sclerotic
prosenchyma and the fibro -vascular bundles may be seen to be
long strands or bands, coursing through the softer fundamental
tissues. •

It should be clearly understood that these three systems are,
in general, not single tissues, but groups of tissues which are
constantly associated together for the performance of certain
functions."^

Microscopic Anato:my (Histology) of the Ehizome.

General Account. Microscopic study of thin sections of the
rhizome shows the various tissues to be composed of inmimerable
closely-crowded cells, which dilfer very widely in structure and
in function. In studying these cells the student should not lose
sight of the fact that they are objects having three dimeusions,
of which only two are seen in sections. And hence a siugk' sec-
tion may give an imperfect or entirely false impression of the
real form of the cells, — just as the faceof a wall of ma^^oury may
give only an imperfect idea of the blocks of which it is built.



* This classification of the tissues is only a matter of convenience, and ha*
little scientific value. B\^ many botanists it has been rejected alto^«'ther ; but
no apology for its use need be made by those who. like the authors, have
found it useful, so long as it is defended by Sachs (\v)io first introduced it) and
its value for beginners is conceded by De Bary.



116



THE BIO LOO Y OF A PLANT.



Por tliis reason many of tlie cells cf^n only be understood by a
â– comparison of transverse and longitudinal sections, and these
should be studied together until their relations are thoroughly
mastered.

The following table gives brief definitions of the leading
vegetal tissues and is good not only for Pteris but for all
plants : —

PRINCIPAL ADULT VEGETAL TISSUES.



Tissues.


Characteristics.


1. Epidermia.


Cells in a single layer covering the outer surface.


2. Pai'enchyma.


Masses of ceUs, rounded, prismatic or polyhedral, usually incom-
pletely joined at the angles, thus leaving intercellular spaces.
Not much longer than broad. Thin-walled.


3. Prosenchyma.


Cells elongated, typically massed, without intercellular spaces.


4. Sieve-tubes.


Cells elongated, thin-walled, panelled with perforated areas,
containing proteids.


5. Ti^acheids.


CeUs thick-walled, elongated, pointed, hard ; walls pitted ; filled
with air.


6. Trachece or
vessels.


Cells very slender, elongated, opening into one another at their
ends, often spirally thickened, and filled with air.



These six tissues are not only found in the rhizome, but ex-
tend throughout the roots and the fronds as well. Moreover,
all the tissues not only of the fern but of all higher plants are
varieties of them.

Special Account. It must not be forgotten that the differences
between tissues are only the outcome of the differences between
their component cells (p. 13). So that the study of the histology
of the rhizome, even if preceded (as it may well be) by a dissec-
tion, and a naked-eye examination of some of the tissues, event-
ually resolves itself into the careful microscopic study of the
several kinds of cells composing those tissues.

The mature parts of the rhizome contain at least nine very
different kinds of cells, the characteristics and grouping of
w^hicli are shown in the following table. In the apical buds,
liowever, this arrangement disappears, and all the cells appear
-closely similar.



HISTOLOGY OF THE lilllZOME. 117

MINUTE ANATOMY OF THE RHIZOME OF PTKIUS AQUILINA.



System.



I. Epidermal



Tissues.



1. Epidermis.



II. Funda-
mental,



III. Fibro-
vascular.



3. Fundamental
parenchyma.



CHAUACTEUISTK.S.



Cells polytronal in crftss-softion, cinijfv. Walls
hard, tliickened, especially towards the outsiil.-
(Fig. 49.)



Cells rounded or poly^ronal in fross-sortion. color-
less. Thin-wallcd, coiitniiiinu prutopluMn, im.
ck'us and starch. Iiitercellulur spaces prescrn

(Fig. 5:-', /.p.)



•^olprntin Ttnr Cells polygonal or semi-fusiform in section, ncarlv
* tSumn empty. No interc-ellular spaces. Walls har-l

cncnyma. ^^^ brown, thickened. (Fig. 49.)



4. Sclerotic pios-

enchyma (or Cells fusiform, empty. "Walls thick, red. (Fig. 5(1. i
sclerenchyma)



5. Wood - par en
chyma.



Like the fundamental parenchyma, but with more
elongated cells. (Figs. 52, .53.)



6. Phloem-paren-
chyma.



7. Phloem-prosen-
chyma, or
bast-fihres.



8. Sieve-tubes.



9. Tracheids (lad-
der-cells).



10. Trachece or ves-
sels (spiral).



Precisely like 5, differing only in position.



Cells fusiform, rich in protoplasm, colorless. Walls
thick, soft. (Figs. 52, 53.)



Having the ordinary characters (see preceding
table). (Figs. 52-5t.)



Pits transversely elongated (scalariform). (Figs.
53-53.)



Very slender, with one or two internal spiral thick-
enings. (Fig. 53.)



Besides the above-mentioned tissues, the rliizonie contains
certain other secondary varieties whicli will be descri])cd further
on.

Epidermal System. Epidermis. It is the fuiu'tiou <>f the
epidermis (aided in this case by the underlying sclerotic ])arc'n-
chyma) to protect the inner tissues from contact with thi' soil
and to guard against desiccation of the rhizome during ch-oiights.
The cells (Fig. 49) are dead and empty, with enormously thick,
hard walls perforated by numerous branching canals. The outer
wall is especially thick.

Fundamental System. The tissues of this system form the
main body of the plant, and in the fern have two widely ditTcr-



118



THE BIOLOGY OF A PLANT.




Fro. 49.— Section showing the epidermis (ep) and the underlying sclerotic paren-
chyma (.s.p) of the rhizome of Pteris aquiUna. Canals, sometimes branching, are
everywhere seen. These served to keep the once-living cells in material con-
nection.




Fig. 50.— Cross-section of sclerotic prosenchyma of the rhizome of Pteris aquilina.
The enormously thickened walls consist of three layers, are perforated by canals,
and are lignified or turned into wood.



HISTOLOGY OF THE RHIZOME.



119



ent functions. T\\q fu7ida7ne7ital parenchytna is a kind of store-
house in wliicli matter and energy are stored — inainlv in tlie
form of starch, CoHio^^b — ^^^^^ in wliich active chemical cliani::e8
take place. The cells are thin-walled and soft, and are ratln-r
loosely joined together, leaving numerous intercellular spaces
(Figs. 52, 53). They contain protoplasm and a nuck'us, and
very numerous rounded grains of starch. This starch is sturud
up by the plant during the summer as a reserve supj)ly of fo(»d
— just as hibernating animals store up fat in tlieir 1)odies for use
during the winter. Accordingly, starch increases in (juantity
during the summer and decreases in the spring when the j)laiit
resumes its growth, before the leaves are unfolded. The j)aren-
chy ma probably has also the function of conducting various sub-
stances (especially dissolved sugar) through the plant by diffusion
from cell to cell.



The sclerotic parenchyma and sclerotic prosenchyma (Figs. 49, 50) are
dead, and hence play a passive part in the adult vegetal economy. The
former co-operates with the epidermis ; the
latter probably serves in part to support the
soft tissues, and to some extent affords a
channel for the conveyance of the sap. The
sap, however, does not flow through the
cavities, but passes slowly along the sub-
stance of the porous walls. The cells of
both these sclerotic tissues have very thick,
hard, brown walls, perforated here and
there by narrow canals. The cells of the
parenchyma are prismatic or polyhedral ;
those of the prosenchyma elongated, and
pointed at their ends. In both, the proto-
plasm and nuclei disappear when the cells
are fully formed. Towards the apical buds
both fade into ordinary fundamental paren-
chyma.

Fibro-vascular System. The Jihro-
vascular bundles (p. 115) are long pio. m. (Aft.^rSarhs.)-viow of
strands or bands of tissue which ap- ^he rhizome, -'7'; /^/[^J'^T;^

*â–  to be transparent so as to snow
pear in cross-section as isolated spots the network of tlie upper nbro-

aT 4 o\ rm 1 n „ ,, ^i. vasciihir biiiunes. /, a leaf,

ng. 48). The bundles are not

really isolated, how^ever, but join one another here and there,

forming an open network (Fig. 51), which can only be seen in a




120



THE BIOLOGY OF A PLANT.



lateral view of tlie rliizome. From this network bundles are
given off which extend on the one hand into the roots and on
the other into the leaves, branching in the latter to form tlie
complicated system of veins to be described hereafter (p. 129).

Eacli bundle consists of a number of different tissues which,
broadly speaking, have the function of conducting sap from one
part of the plant to another.







f IG. 52.— Highly magnified cross-section of a fibro-vascular bundle surround-id by
the fundamental parenchyma, /.p. t, scalariform tracheids ; h.s, bundle-sheath ;
p.s, phloem-sheath ; ?>./, bast-fibres ; s.t, sieve-tubes ; p.p, phloem-parenchyma ;
u\p, wood (xylem) parenchyma; s.i% spiral vessel.

These tissues have the following definite arrangement. Beginning with
the outside of a bundle, we find (Figs. 52, 53) —

1. Bundle- sheath ; a single layer of elongated cells enveloping the
bundle, probably derived from and belonging to the fundamental system.

2. Phloem-sheath ; a single layer of larger parenchymatous cells con-
taining starch in large quantities.

3. Bast-fibres ; soft, thick-walled, elongated, pointed cells containing
protoplasm and large nuclei.

4. Sieve-tubes; larger, soft, thin-walled, elongated cells containing
protoplasm and having the walls marked by areas perforated by numerous
fine pores' (panelled). They join at the ends by oblique panelled partitions
(shown in Figs. 52 and 53).



HISTOLOGY OF TUE RHIZOME.



121



^.Phloem-parenchyma; ordinary parenchymatous cells filled with
starch, scattered here and there among the bast-fibres and sieve-tuljcs.

6. Tracheids {scalariform) or "ladder-cells" ; occupying most of the
central part of the bundle. Their structure calls for some remark. They
are empty or air-filled fusiform tubes, whose hard, thick walls are in the
young tissue sculptured with great regularity into a series of transverse
hollows or pits, which finally become actual holes. The walls of the
tracheid are therefore continuous at the angles, but along their plane sur-



ps. s.t



fp. 1.8.




Fig. 53.— Longitudinal section of a fibro-vascular bundle, surrounded by the fun-
damental parenchyma. />./, bast-flbres; 7>.s, bundle-sheath; /.p, fundauiental
parenchma ; p.p, phloem-parenchyma; y.f^., phloem-sheath; s.f,* sieve-tubes; f,
scalariform tracheids or ladder-cells ; w.i>., wood-parenchyma.



faces become converted into a series of parallel bars, makinc: a grating of
singular beauty. The slits between the bars are not rectangular jiassages
through the w^all, but are rather like elongated, flattened funnels, opening
outwards. The sides of the funnels are called the borders of the pits: and
pits of this sort are called bordered scalariform pits (cf. Fig. 53).

7. Tracheae or vessels (spiral) ; scattered here and there among the
tracheids, and hardly distinguishable from them in cross-section. They
are continuous elongated tubes filled with air, and strengthened by a b<'au-
tiful close spiral ridge (sometimes double) which runs round the inn«T face
of the wall (Fig. 52).

The tracheids and vessels are of great physiological importance, being
probably the main channels for the flow of sap. Sap is water holding
various substances in solution. The water enters by the roots, flows prin-
cipally through the walls of the vessels and tracheids, and not through
their cavities, which are filled with air, and is thus conducted through the
rhizome and upwards into the leaves.

8. Wood-parenchyma ; cells like those of the phloem-parenchyma (5)
scattered between the vessels and tracheids.



122



THE BIOLOGY OF A PLANT.



Branches of the Rhizome These repeat in all respects the
structure of the main stem. Thej are equivalent members of
the underground part, and differ in no wise, excepting in their
origin, from the main stem itself.

Roots. The roots may easily be recognized by their small
size and tapering form, and their lack of the lateral ridges of the




Fig. 54. (After De Bary.)— Sieve-tubes from the rhizome of Pteris aquiJina, show-
ing: A, the end of a member of a sieve-tube ; B, part of a thin longitudinal sec-
tion. The section has approximately halved two sieve-tubes, S> and S^ , which are