32 THE TISSUES OF PLANTS
the pith cells of one or two year old twigs of apple. These are
also somewhat tliick-walled.
41. Sclerenchyma is the name given to a tissue con-
sisting of more oi; less rounded or polyhedral, usually
not much elongated, thick-walled cells whose function is
to give support or protection to other tissues. These
cells originate from meristem by the thickening and
lignification of the walls, passing through an intermediate
parenchymatous stage. During the process numerous
spots on the walls remain thin so that
eventually they show as canals from the
small central lumen of the cell to the
original outer wall. These canals or pits
Fig. 11. -Sclerenchyma COrrCSpOUd ITi adjaCCnt CClls. Upon
( 10 -ory nut). reaching their final development the cell
contents die. Sclerenchyma cells are often called stone
cells. They are found in seed coats, nut shells, bark,
etc., where protection or support is required.
42. Of a much different type from the foregoing are
those tissues consisting of elongated cells with more or
less thickened walls whose function is the mechanical
strengthening and support of the plant body. To per-
mit bending while at the same time retain-
ing their supporting function they are more
or less elastic, a characteristic less marked
in the short-celled sclerenchyma whose func-
tion is protection or only local support. ^^^^ 12 — Coiien-
We can distinguish two types of these sup- chyma.
porting or mechanical tissues, collenchyma and fibrous
tissue.
43. Collenchyma. Directly ])eneath the epidermis of
many plants are found smaller or larger strands of elon-
gated cells whose longitudinal cell walls are thickened at
the angles where three or more cells come in contact.
FIBROUS TISSUE 33
Except in old cells the thickening rarely extends out
upon the wall lying between the angles. The cells
remain alive, for a long while, and usually contain chloro-
plasts. They remain capable of growth longitudinally.
Accordingly collenchyma is found to be the chief mechan-
ical tissue in growing parts of plants, such as stems,
leaf-stalks, etc. The thickened parts of the walls are
composed of cellulose and transmit the light with a pecu-
liar pearly luster when viewed in cross-section, the lumen
of the cell under these conditions appearing darker than
the cell walls.
44. Fibrous tissue consists of elongated cells, thick-
ened on all sides, usually overlapping at their more or
less tapering, often pointed, ends. The walls show
minute, usually ol^liquely placed, slit-
like pits. After they reach full develop-
ment, the cell contents die, so that the
cells are incapable of further growth or
development. The thickened walls are
usually strongly lignified. In cross-sec-
tion the cells are round or by mutual Fig. 13.— Wood and
, 1 -r-,., . . bast cells.
pressure, angled. Inbrous tissue is
found as the chief mechanical tissue in parts of plants
which have completed their longitudinal growth. Two
types can be distinguished, viz., bast and wood fibers.
The former are located in the outer part of the stem
(in the cortex in the Dicotyledoneae), the latter in the
true wood. Bast fibers are usually longer than wood
fibers, and more slender, with often thicker but less com-
pletely lignified and hence more elastict walls. Their
usual length is from 1 to 2 mm. but in Bochmcria nivea,
the ramie plant (according to Haberlandt) they reach a
length of 220 mm., the longest plant cells known. Wood
fibers are usually shorter (mostly 0.3 to 3.1 mm.) often
34 THE TISSUES OF PLANTS
somewhat thicker, with less tapering ends and frequently
with less thickened walls which are more strongly ligni-
fied than those of bast fibers.
Laboratory Studies, (a) Break tlie shell of a hickory nut,
ahnond, coconut, walnut, peach-stone, etc., and after smooth-
ing the broken surface, cut off a thin shaving, using a pocket
knife or scalpel held at rather an oblique angle. Mount in
water and a httle potassium hydrate. The very small cell
cavities show connecting pits or canals radiating from them to
the original cell wall where they meet similar canals from the
centers of adjoining cells, being separated only by the thickness
of the original wall. Concentric markings are visible in the
cell walls in some cases.
(b) Determine whether the walls in sc^erenchyma are made
of cellulose or are lignified, by testing one section with a 5 per
cent, aqueous phloroglucin solution followed by hydrochloric acid
which gives a red color for lignified walls, and another section
with iodine solution followed by somewhat diluted sulphuric
acid which gives a blue color for cellulose walls.
(c) Sclerenchj^ma may be found and studied (1) as the little
''grit" bodies in the flesh of the pear or sapodilla (Achras
mpota), (2) in the underground stem of the brake {Ptcridium
aquilinimi), (3) next to the epidermis in the prickly pear
(Opuntia), as well as (4) in coats of many seeds, e.g. apple,
squash, wild cucumber, and (5) forming the body of the
seeds of many palms, e.g. date.
(d) Examine a young leaf-stalk of the squash or pumpkin
and note the whitish bands, 1 or 2 mm. wide, which extend from
end to end just beneath the epidermis. These are bands of
collenchyma. They may be readily torn out, when the stalk
will be found to have lost much of its strength.
(e) Make a very thin cross-section of the leaf-stalk of one of
the foregoing plants, exactly at right angles to the axis of the
collenchyma strands, and examine under low and high magnifi-
cations. Test with iodine and sulphuric acid to determine the
composition of the walls.
(/) ]\Iake longitudinal sections through these collenchyma
Imnds. If good sections are obtained the thickened angles
(becoming thin toward the point where the thin cross walls
occur), chloroplasts and nuclei will be found. However, only
TRAClIEAllY TISSUE 35
those cells that happen to be so placed that a thickened angle
appears in the section will show this feature. On the other
liand, if the section passes between the corners of the cell the
walls will appear thin.
(g) CoUenchyma may be found also in the young green shoots
of elder (Sanibucus) and some other shrubs, in the stems of
lamb's quarters (Chenopodium), pigweed (Amaranthus),
petioles of beets and very many other plants.
(h) Make thin longitudinal sections of the wood and bark of
the basswood (Tilia) or maj)le (Acer) and macerate, to
separate the cells, in Schulze's reagent (i.e. heat in a test tube in
nitric acid to which has been added a little potassium chlorate).
Mount a bit of the macerated wood section on a slifle and tap
the cover glass, or tease the section apart with needles.
Study the wood fibers. Do the same for the bast fibers in the
bark.
(i) Now make thin longitudinal and cross-sections of the
same kind of twig without macerating and study the fibers in
place to note the relation of the overlapping cells. In the cross-
section, note the appearance of the fibers and their position in
the twig.
45. Besides the foregoing, there is a group of tissues
which have as their chief function the conduction of
water and food, the so-called conductive tissues. These
are of three kinds: tracheary tissue, whose primary func-
tion is the transportation of water, and sieve and lat-
iciferous tissues, which are chiefly concerned with the
conduction of food substances manufactured by the
leaves.
46. Tracheary tissue is of many kinds. The term is hero
used to include those elongated cells, whose chief function
is the transport or storage of water. The lumen is usually
rather large with the wall thickened in a more or less regu-
lar manner to give strength. At the same time, a consider-
able portion of the wall remains thin, permitting the en-
trance or exit of water. The cells are not living, i.e. their
protoplasm dies as soon as they have attained their final
36 THE TISSUES OF PLANTS
development, so that the conduction of the water is not
dependent upon the activity of these cells but occurs in
the cavities left empty by the disappearance of the proto-
plasm. Since the cells lack protoplasmic contents which
would furnish the turgor to keep them from collapsing,
the thickening of the walls is necessary. It often happens
that adjoining living cells swell out through the thinner
places into these cells, these bladder-like projections
being called tyloses. A distinction is made between
tracheids which are formed of single cells, and tracheae
(singular, trachea) or vessels, which are more or less
elongated tubes formed by the absorption of the cross
walls of adjoining cells so that the lumens of many suc-
cessive cells are all connected. The latter usually attain
the greater diameter. Tracheids are mostly not over 1
mm. long although in some cases they reach a length of
1 centimeter or even much more. Tracheae, accord-
ing to Strasburger, average about 10 centimeters long,
but in some cases reach a length of 2 to even 5 meters.
In some vines, the diameter reaches 0.3-0.7 mm. Trach-
eary tissue is found only in the higher plants, i.e., Seed
Plants and Ferns and Fern Allies.
47. In accordance with the character of the thickening,
there may be distinguished sev-
eral types of tracheary tissue,
these same types of thickening
being found both in tracheids
l^^)Mg?"^r^ I M rn ^^^ tracheae. These are ringed
^^ringtdTspirJureticJiatedK'^^ (or anuular), Spiral, reticulated
(netted), scalariform (ladder-
like) and pitted tracheae or tracheids. All but the last
are named after the manner of the internal thickenings of
the walls. The pitted cells, however, are those in which
the thickening is more extensive than in the others, the
TRACHEARY TISSUE 37
thin places remaining only as small pits. The cells of all
these structures are usually more or less pointed and over-
lapping at the ends, except in some of the tracheae in
which the square end walls were dissolved out. They
are mostly round or by mutual pressure somewhat angled
in cross-section.
48. The spiral and annular thickenings are the
only types found in the tracheary tissue that is formed in
stems or roots that are still elongating, as it is possible
for such cells to elongate by the stretching or growth of
the unthickened portion, whereby the rings become
farther apart or the spirals stretched out at a greater
angle. Very often adjacent rings may be connected here
and there by a spiral or the same vessel may have annular
thickenings in one part and spiral in another. There
may be from one to three or four spirals. The reticu-
late type of thickening is perhaps to be considered as a
many-spiraled type with numerous cross connections
from one spiral to the next so as to form a network.
Scalariform vessels are usually angular in cross-section
and have their thickenings on the flat faces of the prisms
as horizontal bars connected to the somewhat thickened
angles, and leaving horizontally elongated thin areas be-
tween them like the openings between the rungs of a
ladder. All transitions may be found from the reticu-
lated or scalariform structure to the pitted type. The
pitted tissues are of two types: (a) with simple pits, and
(6) with bordered pits. In the first the pits are of the
same diameter through their whole depth or even wider
toward the center of the cell. In the second, the}- are
narrow, adjacent to the cell lumen and are much wider as
they approach the middle of the cell wall, the cavity of
each pit having the shape of a planoconvex lens. The
wall or diaphragm separating the adjacent pits of ad-
38
THE TISSUES OF PLANTS
xoi
Fig. 15. — Tracheary tissue
(pitted and tracheids).
joining coUrf is very thin and permeable to water except
a button-like thickening, in the center. When seen in
surface view, a bordered pit shows a double circle, the
smaller inner one being the opening into the pit and the
outer circle, the outer edge of the diaphragm.
49. Special mention must be made of the tracheids of
Conifers (Spruces, Pines, etc.).
These are shaped and thickened
like wood fibers but differ in
possessing on their radial faces
one or more longitudinal rows
of bordered pits. They com-
bine the functions of tracheids
and fibrous tissue, serving both
for conduction of water and for
mechanical support.
50. Sieve Tissue. In almost
all of the higher plants and in many of the more massive
lower plants, there are found rows of elongated rather
wide cells whose transverse separating walls are pierced
by numerous larger or smaller perforations. Where two
such cells lie side by side parts of the lat-
eral separating wall will often show simi-
lar perforated areas. These are the so-
called sieve plates which give the name to
this tissue. The walls of the sieve tubes,
as the elongated cells are called, are usu-
ally rather thin. The sieve plates, on the
contrary, are rather thick. In surface view
they look like a sort of network. In some cases, the
meshes of the net are perforations, in others, they are
thin walled areas perforated by several to many fine holes.
The mature sieve tubes have the walls lined with a thick
layer of cytoplasm in which the nucleus is imbedded.
Fig. 1G.— Sieve
tissue.
LACTICIFEROUS TISSUE 39
The centi'iil vacuole is filled with a liquid ver}- rich in i)ro-
tein matter, the masses of this protein substance often
being continuous through the pores of the sieve plates
with those of the adjoining sieve tubes.
51. The sieve tubes of the Flowering Plants are
accompanied b}- usually slender parenchyma cells full of
protoplasm, the so-called companion cells. The walls
between these and the sieve tubes are perforated by
numerous very minute passages invisible except b}" special
manipulation. Other forms of parenchyma cells are
usually found adjacent to the sieve tissue. The function
of the sieve tissue is probably the transportation of
protein substances from the leaves to parts of the plant
where they are needed in the construction of new cells.
Possibly, also, sugars are transported, at least in part, in
the same tissues as well as in the ordinary parenchyma
cells near them. The function of the companion cells
is not certain.
52. Laticiferous Tissue. This consists of a system
of tubes extending throughout the plant
and filled wdth a substance called latex.
This is usuall}' white (hence the name ''milk
tissue" often applied to this kind of tissue),
but may be colored red, j^ellow or even be
almost clear and colorless. The latex con-
sists of water containing usually much pro- Fig. i?.— Laticif-
,, erous tissue.
tem matter as well as some sugar and
salts dissolved in it, and holding in suspension numerous
minute globules of resin or in many cases, caoutchouc.
On exposure to the air, the latex often coagulates. It is
from the latex of many plants that rubber and gutta
percha are obtained, while other substances of great value
are often found in it also, e.g. opium in the latex of
the poppy. In some plants, starch grains are found in
40 THE TISSUES OF PLANTS
the laticiferous tubes. The walls are lined with cyto-
plasm containing nuclei. They are mostly thin but in
Euphorbia the walls are thick and elastic.
63. Two distinct types of laticiferous tissue may be
distinguished: (1) Non-anastomosing and (2) Anastomos-
ing. The forrner consists of branching tubes which
originated from single cells in the embryo. These cells
elongate and branch, keeping pace with the growth of the
plant, forcing their way between the meristem cells
exactly as if they were part of a fungus instead of a tissue
of the plant in which they occur. They appear never to
anastomose. They are found in the Euphorbiaceae,
Moraceae, Apocynaceae, etc., i.e. in the chief rubber-
producing families.
54. The anastomosing milk vessels are formed by
the fusion (that is through the resorption of the separat-
ing walls) of adjacent meristem cells in such a way as to
form a network of latex-bearing tubes. Short lateral out-
growths may also be sent out from one tube to another,
thus increasing the number of anastomoses. Laticiferous
tissue of this type is found especially in theLactucaceae,
Papaveraceae,etc., as well as in a few of the Euphorbiaceae,
e.g. Manihot and Hevea, both rubber-producing trees of
great economic value.
Laboratory Studies, (a) Make a thin longitudinal section
of the stem of garden balsam (Impatiens) or any other her-
baceous plant that has not begun to become thickened and
wood3^ The section should pass through one of the vascular
bundles. There will be found various tj^pes of tracheary
tissue, those facing the interior of the stem being usually of the
annular or spiral type, with reticulated and pitted types to-
ward the outside.
(6) Good plants for study are Tradescantia, especially for
ringed and spiral types of tracheary tissue; Sida, for good spiral
and reticulated types; Indian corn, pumpkin or squash, etc.,
for large pitted vessels.
LABORATORY STUDIES 41
(<:•) Study tlie foregoing types of tracheary tissue in cross-
section in comparison with the longitudinal sections.
((/) The larger pores in the wood of oak, hickory, etc., as
well as in the grape, are pitted vessels.
(e) Excellent scalariform vessels are to be found in the
leaf-stalks or better still, in the underground stems of the
brake {Ptcridium aquilinum).
(/) The tracheids of pine, spruce, etc., resembling wood fibers
in shape, but with bordered pits, should be studied by making
tangential and radial longitudinal sections as well as cross-
sections of the wood. The bordered pits occur only on the
radial surfaces of the tracheids.
{g) Spirally marked tracheids, similar in shape to the fore-
going, may be found in the wood of the hackberry (Celtis),
and ash.
{h) By treating various kinds of wood with Schulze's reagent
(nitric acid and potassium chlorate, warmed) the various cells
will be separated and the tracheary elements of different kinds
can be studied separately.
{%) Sieve tissue is easily found by making longitudinal sec-
tions of the stems of squashes or pumpkins (Cucurbita) or
other vines such as grape, clematis, hop, etc. They will be
found in the part of the vascular bundle Ij'ing toward the
outside of the stem and in the case of Cucurbita also on the
inner side. By staining with eosin or carmine, the protoplasm
and protein contents will be stained. If alcoholic material be
used, the contents will be found shrunken away from the sieve
plates. If portions of living stems are killed before sectioning
by dipping into very hot water, the protein and protoplasmic
contents will be coagulated without much contraction.
ij) Make numerous very thin cross-sections of the same
stems and examine until sieve plates are found and studied in
surface view.
{k) Examine a drop of latex from milkweed, spurge or poppy,
under high magnification. The suspended granules will be
visible as fine dark brown bodies by transmitted light. Test
with iodine to determine whether starch grains are present.
(/) Collect a quantity of latex of spurge (Euphorbia) and
let it evaporate in a watch glass. The residue is a sticky,
rubbery mass, which on being burned, has the characteristic
odor of burning rubber.
42 THE TISSUES OF PLANTS
(m) For the study of laticiferous tissue thin tangential
sections are best. The tissues will show as tubes filled with a
brown granular mass, the latex. The non-anastomosing type
can be found in the milkweed (Asclc})ias), dogbane (Apo-
cynum), and spurge (Euphorbia), especially the more fleshy
forms of the latter. The anastomosing tyj^c can l)c studied
in the petioles of dandelion or lettuce, or in the stem of the
poppy.
(n) The long, branching, non-anastomosing laticiferous tubes
of Euphorbia can be isolated from the more fleshy leaved sorts
by boiling the leaves in dilute potash solution and then teasing
out a piece of the softened tissues.
(o) To examine the tissues in situ, the leaves should be
placed in strong alcohol (90-95%) for some hours. If the
leaves are thick, thin sections should be made parallel to
the surface. These sections, or the whole leaves if they are
thin, should then be placed for an hour or so in a clearing fluid
made of equal parts of turpentine and carbolic acid (phenol).
Mount the section or leaf in this fluid. The tissues are made
transparent, and the laticiferous tubes filled with granules of
latex can be studied with great ease. The same method can be
used for studying both types of laticiferous tissue.
REFERENCE BOOKS
The books enumerated for Chapter I and the following.
A. DeBary, Co?nparative Anatomy of the Vegetative Organs of
Phanerogams and Ferns (Engl. Ed. 1884. Oxford).
G. Haberlandt, Physiologische Pflanzenanatomie, Leipzig,
1904. (Engl. Ed. 1914. London.)
CHAPTER III
GROUPS OF TISSUES, OR TISSUE SYSTEMS
HISTOLOGY
65. In the lower plants, where all cells are essentially
alike and no distinction of tissues can be made, we often
find that growth takes place in all parts of the plant, al-
most every cell being capable of growth and division at
any age. In many plants, however, in which the differ-
entiation into various kinds of tissues is still almost lack-
ing, we find that growth is more or less limited to certain
regions of the plant. In those plants where the tissue
differentiation is strongly marked, we find that the
formation of new parts, as well as growth, is localized
in groups of meristem cells at the apices of stems and
roots (and also in many plants at the nodes), the older
cells of these groups gradually changing into the more
permanent tissues of the plant.
56. In many seaweeds and fungi, where the plant
body consists of separate or adjacent rows of cells, the
terminal cell of each row elongates and divides by a
cross partition and perhaps division occurs in one or two
cells behind it. Except for the formation of branches,
longitudinal divisions may be lacking and the result is
only the formation of rows of cells.
57. In the plants which are not so markedly fila-
mentous in structure the new tissue at the ai)ex may arise
by the division of a single aj)ical cell. This division
may be by horizontal i)artitions, the seguKMits thus
43
44 GROUPS OF TISSUES, OR TISSUE SYSTEMS
formed dividing subsequently by both horizontal and
longitudinal partitions (as in Sphacelaria and many other
algae). More often, we find that the apical cell is a three
sided pyramid, the convex base of the pyramid being
the apex of the shoot. Successive cells are cut off from
the three sides and the segments
thus produced divide by various
partitions so as to produce the mass
of meristem cells from which the tis-
,„ . . , „ r sues become differentiated. Some-
FiG. 18. — Apical cells of , ,
a seaweed (Sphacelaria), timeS, mstcad of the apiCal Cell
and a moss. . ^ ,
cuttmg off three rows of segments,
it produces only two or in other cases, four.
58. In most of the Flowering Plants, a group of cells
is found at the apex of the stem or root instead of one
cell, these giving rise, by their division, to the mass of
meristem. This group of apical cells, or the single apical
cell with the cells derived from it, is called the growing
point.
59. We can usually distinguish three different tissue
regions at or a short distance back from the growing
point of higher plants. At the outside we find a single
layer, the epidermis, which consists of cells that divide
only by walls perpendicular to the surface. When this
layer has an initial cell or cells distinct from the inner
layers the portion near the apex is often spoken of as
the dermatogen. The next region is spoken of as peri-
blem, and may consist of one or several layers of cells
surrounding the centrally located plerome. These two
regions may have separate sets of apical cells or the dis-
tinction may occur only some distance from the apex.
In most roots, the apex is covered by the root cap, a
mass of cells produced by the periclinal division (i.e.
by walls parallel to the surface) of a layer of cells outside
GROWING POINT 45
of the dermatogen, or in some cases, of the dermatogen
itself, or, in still other cases, by the division of some of
the cells of a common mass of initial cells from which the
root cap as well as epidermis, periblem and plcrome
arise. On the growing points of stems, the new branches
arise by the formation of secondary growing points at
the side of the main one, these having the same
general plan. Those that produce the leaves often grow
faster than the mai growing point and sur-
round and protect it, thus forming a bud. v ^ .â–