(CeHisOe), etc.
29. Cane sugar is found in great quantities in the cell
sap of the sugar cane, sugar beet, sugar maple, sorghum,
Indian corn and many other plants. The first two plants
produce the bulk of the sugar of commerce. In many-
fruits, such as grapes, cherries, gooseberries, figs, etc.,
glucose is present, while in still others, e.g. pineapple,
peach, plum, strawberries, etc., the two are mixed.
Fructose, as the name implies, is found in many fruits,
e.g. the grape. In many, if not in most plants glucose
seems to be the form in which green cells manufacture
their food, storing up the excess over immediate consump-
tion usually as starch, from which it is again obtained as
glucose. Inulin is found mostly in plants of the sunflower
family, e.g. sunflower (Helianthus), Dahlia, elecampane
(Inula), etc.
30. The organic acids found in the cell sap may occur
in acid form, but frequently are found as acid salts of
calcium or potassium or some other base. The most
common of these acids are maUc, citric, tartaric and ox-
alic. They seem to be present in some cases as food for
the plant while in others they doubtless help to keep the
cell turgid by raising the osmotic pressure within the cell
to the proper degree.
31. Among the substances found in the cell sap in so-
lution are certain compounds known as alkaloids. These
are perhaps in some cases products of the breaking down
of more complex substances and to be looked on as a sort
of excretion product comparable to urea in animals.
However, in certain plants they may serve as reserve
food as they are used up by the plant if no other food is
available. They are nitrogenous compounds of compli-
FORMATION OF NEW CELLS 19
cated composition, usually bitter to the taste and very
frequently poisonous to animals.
Laboratory Studies, (a) To show the large amount of
water in living cells place a few threads of pond-scum (Spiro-
gyra) in a little water and examine under the microscope.
Add a httle strong glycerine which has a great avidity for
water. Note how the cells collapse as the water is withdrawn.
Repeat the experiment with thin sections of some herbaceous
stem or simply allow the latter to dry out in the air.
(b) Taste the stem of sugar cane or growing Indian corn or a
piece of a sugar beet. The presence of sugar is readily recog-
nizable. Put small pieces of these plants into considerable
quantities of 95 per cent, alcohol to remove the water, or into
pure glj^cerine. The water is withdrawn rapidly by the
reagents and the cane sugar, which is practically insoluble in
them, crystallizes out in fine stellate crystals Sections for
examination must be mounted in the alcohol or glycerine as
water will redissolve the sugar.
(c) Make thin sections of the root of Dahlia or sunflower
(Helianthus) that has been preserved in strong alcohol and
note the large sphaerocrj'stals of inulin.
(d) To study glucose or fructose test the juices of various
fruits with Fehling's solution, which gives a precipitate of copper
oxide with both these sugars but not with cane sugar or inulin.
(e) The presence of acids or acid salts is readily discernible
by the taste in many plants, e.g. stem of rhubarb, leaves of
Oxalis, fruit of lemon, cranberry, etc. In smaller quantities
it can be demonstrated by placing the cut surface of the tissue
to be tested in contact with a piece of blue litmus paper which
will be turned red by the action of acids.
32. Formation of New Cells. No cell can originate
except from some pre-existing cell or cells. IMost cells
are capable of producing new cells at some stage of their
development, but frequently the power is soon lost.
New cells arise either through the division of a cell or
through the union of two (or rarely more) cells. In the
cell formation by division we distinguish two types, i>ac'h
20
PROTOPLASM AND PLANT CELLS
with modifications, \iz., fissioii, in which the cell divides
into two adjacent parts which may or may not remain at-
tached, and internal cell formation, in which the proto-
plasm within the cell divides into several cells which
eventually escape from the old cell wall as naked cells
(zoospores and motile gametes) or form new walls for
themselves within the old wall and bc^come free on the
rupture or decay of the old wall. The latter type in-
cludes cases in which all the protoplasm is used up in
forming the new cells, as in zoospore formation, as well
as those in which only a part is so used, the remainder
W4
til:::.
Fig. 7. — Kuryokiucsis (mitosis).
lying between the new cells and the old wall, as in the
formation of ascospores within the ascus. Several forms
of fission may be distinguished. The commonest type
is that in which the protoplasm of the cell separates
into two parts that secrete a new wall between them,
the new cells thus remaining attached to each other.
The new separating wall may be formed as a ring-like
thickening on the old wall which gradually increases in
MITOSIS (KARYOKINESIS) 21
width until it has comi)h'ted the separation of the two
protopUismic masses, this being tlie commoner way in
the lower plants, or the wall may be produced sim-
ultaneously at all points at the plane of separation be-
tween the two protoplasts, as is the case in most higher
plants. In some of the lower plants the whole wall be-
gins to constrict at the middle, giving the appearance of
pinching the cell into two separate cells which are then
free from one another. A peculiar type of fission is that
termed budding, in which a small outgrowth appears at a
point on the cell, gradually enlarging until it is as large
as the old cell and then l^ecoming separated from it by
constriction of the wall at the point of emergence. This
is especially characteristic of, but not confined to, some of
the yeasts.
33. Cell division is in most cases initiated by, or more
or less immediately preceded by, the division of the
nucleus. In coenocytes, on the contrary, this connection
between nuclear division and that of the coenocyte seems
to be lacking. Two types of nuclear division may be
distinguished, direct or amitotic and indirect or mitotic.
The latter process is generally known as mitosis or karyo-
kinesis. The direct division is comparatively rare and
appears to consist of a simple pinching in two of the nu-
cleus. By far the commonest method is that of mitosis.
This is a very complicated process and is essentially as
follows, ])eing subject, however, to many more or less
pronounced variations in different plants. If a centro-
some is present, which is apparently the case only in some
of the lower plants, it divides into two centrosomes which
move around outside the nucleus until thej- lie at oppo-
site sides in a line at right angles to the plane of division.
The nuclear reticulum now begins to resolve itself into a
fine tangled thread without cross connections, the chro-
22 PROTOPLASJ^r AND PLANT CELLS
matin granules spreading themselves out along the thread
until it is of even thickness. The thread rapidly shortens
and thickens, eventually becoming a thick, more or less
distinctly spirally arranged thread (spirem stage). At
the same time the nucleolus has been growing smaller or
less distinct and soon disappears entirely. In the spirem
thread there often becomes visible at this stage a split for
its whole length. However, it does not separate along
this split as yet. In the mean time outside the nucleus
there begin to appear in the cytoplasm immediately
surrounding the centrosomes fine lines, or fibrillae (of
kinoplasm) , which appear to center at the centrosome and
extend from it in all directions but especially toward the
nucleus. In the plants which have no centrosomes there
appear near the poles of the nucleus tangled masses of
fine fibrillae which in some cases form a sort of cap at each
pole or even may entirely surround the nucleus. From
this tangled mass the fibrillae gradually untangle them-
selves somewhat and finally lie in the form of a cone at
each pole, with the apex away from the nucleus. In the
forms with centrosomes one of the latter lies at each apex,
often surrounded by radiating fibrillae which may reach
out even to the cell wall. Where the mass of fibrillae
comes in contact with the nucleus the nuclear membrane
disappears and soon after vanishes at all other points
also. The fibrillae push into the nuclear cavity. In the
meanwhile the spirem thread breaks transversely into a
number of segments called chromosomes, the number
being constant for all vegetative nuclei of a given species
of plant. Two sets of kinoplasmic fibrillae can now be
recognized. Some push through the nuclear cavity until
they meet and unite with similar ones from the other pole,
forming a spindle-shaped structure commonly spoken of
as the nuclear spindle. Other sets of fibrillae push toward
MITOSIS (KARYOKINESIS) 23
the chromosomes and become attached to them, one or
more sets from each pole being fastened to each chro-
mosome. In some way, perhaps by the contraction of
these fibrillae, the chromosomes are brought to he at the
equator of the spindle, forming the so-called equatorial
plate. The chromosomes are of various shapes, like rods,
or resembling the letters J, V or U, more frequently the
last two. Usually the faint longitudinal split which
first became visible during the spirem stage is quite dis-
tinct. As the fibrillae attached to the chromosomes con-
tinue to contract the latter are torn in two along the line
of this longitudinal split, one half being dragged toward
each pole. When these daughter chromosomes, as they
are called, reach the two poles they soon join to each other
end to end and form spirem threads similar to those
formed before the cleavage into chromosomes (the di-
spirem stage). These elongate and finally form a long
tangled thread along which the chromatin begins to
assemble in lumps and which soon forms short lateral
connections to make the typical nuclear reticulum. In
the meantime the nuclear membrane has appeared
around each daughter nucleus and the nucleolus has made
its appearance. The kinoplasmic fibrillae around the
centrosome gradually disappear in the plants with cen-
trosomes, while in plants without centrosomes they dis-
appear in about the same way that they appeared, or in
the higher plants take part in the formation of the sepa-
rating membrane. In this latter case the spindle fibrillae
seem to increase in number until they occupy the whole
width of the cell. At the equatorial plane a little knot
appears on each fibrilla. The fibrillae contract and as
they shorten the knots increase in size until by the con-
tact of the knots with each other a thin membrane (of
kinoplasm) is formed which separates the protoplasm of
24 PROTOPLASM AND PLANT CELLS
the coll into two parts. This membrane splits and be-
tween these two plasma membranes is secreted the first
layer of the cell wall (middle lamella). It is of interest
to note that mitotic nuclear division is essentially the
same in animals and plants. In the former, however,
centrosomes are usually present while they are lacking in
plants except in some of the lower groups.
34. In internal cell formation the nucleus usually
divides several times before the cytoplasm separates.
Usually the new cells are formed almost simultaneously
in this case. In many cases the cleavage of
the cytoplasm is such that all of it is used up
in forming the new cells, the spindle fibrillae
taking no part in the process. In other cases,
as in the formation of ascospores in the ascus,
the kinoplasmic fibrillae radiating from the
Internal c^ ccutrosomc outlinc the new cell in the midst
formation. rj.i i?j.i i ' ^ n
01 the mass oi cytoplasm, leavmg much of
the latter outside of the new cells, the so-called cpiplasm.
35. Cell formation by union is in the main the opposite
process to that by division. The union of the cytoplasm
of the uniting cells is usually followed by the union of the
nuclei to form one nucleus. If the cells are naked the
process is comparatively simple, but when enclosed in
walls the cells must either escape before uniting, or open-
ings must be made in the walls so that one cell can pass
into the other. By the union of the two nuclei the num-
ber of chromosomes is doubled and remains at this so-
called diploid number until by a peculiar modification of
the mitotic process (the reduction division ormeiosis) the
number is reduced to the original (or haploid) number.
Laboratory Studies, (a) Scrape off, after moistening with
alcohol, a little of the 3'oung white moldy growth on a lilac
leaf (powder}' mildew) or of similar mildews on cherry shoots
_ >EKrr UBRARf
IJ^ C. State College
LABORATORY STUDIES 25
grass leaves or other plants. Mount in dilute potash.
Threads will be found showing the formation of new cells
(spores) l\v fission.
(b) Add a little sugar (preferably glucose) to a little potato
water (made bj^ grating up a raw potato and heating with
water to extract the soluble matter and filtering) and break up
in it part of a yeast cake (''compressed yeast") setting the
solution in a warm place. Examine a small drop of the scum
or sediment after a few hours for cells showing the type of
fission called budding.
(r) By growing yeast for a few da3\s on a moist slab of
plaster-of-Paris under a bell jar or, less successfully in many
cases, on the cut surface of a raw potato or carrot some of the
cells may be found to have produced four cells by internal cell
division.
(d) Make a very tliin cross-section through a young flower
bud, or moss capsule. In the stamens of the former or in the
interior of the latter, if they are at the right stage, will be found
cells which have divided internally into four parts which sub-
sequently become spores, each with a thick wall of its own.
(e) Take a flower bud of Tradescantia just before opening
and remove a stamen and mount in water of about the room
temperature. By examining with proper manipulation of the
light, some cells near the tips of the stamen hairs may be found
in division and the main features of the mitotic division of the
nucleus may be dimly seen.
(/) Examine specially prepared, stained sections of rapidly
growing root tips, stamens, etc., where cell divisions are taking
place frequently. Find and study as many stages as possible
of the mitotic division of the nucleus and cells. These prep-
arations require especial technique and cannot be made
successfull}^ by the beginning student. It is desirable that he
study good preparations. Such can be obtained of various
su})ply houses if the teacher has not the time or desire to make
them.
(g) Cell formation by union can be observed in the conjuga-
tion of pond scums (Spirogyra or Zygnema) or of black molds
(Mucoraceae, especially Sporodinia, which is frequent on
decaying toadstools and can be transferred to bread where it
grows luxuriantly).
26 PROTOPLASM AND PLANT CELLS
REFERENCE BOOKS
B. AL Davis, Studies on the Plant Cell (American Naturalist,
(1904-1905, Boston).
Strasburger, Jost, Schenck and Karsten, Lehrhuch der
Botanik, 11 Ed., Jena, 1911 (or English Edition), and the
12 German Ed. 1913.
CHAPTER II
THE TISSUES OF PLANTS
HISTOLOGY
36. In many groups of plants a single cell makes up
the whole plant. In such groups the cells may vary
considerably in different species but there is not possible
a differentiation into cells of different structure for differ-
ent functions. All of the normal activities of the plant
are carried on by the same cell and, therefore, the modi-
fications of the cell are limited to those that do not inter-
fere with any of these functions. Aside from these
limitations the cell may vary much in size, shape, struc-
ture of wall, location and size of nucleus and vacuoles,
etc.
37. In other forms of plants there are several to many
cells forming one plant in which all of the cells are
essentially alike and each capable of continued existence
by itself even if the others should be destroyed. Such a
plant is scarcely more than a group of nearly independent
individuals. As we study the more and more complex
forms of plants, however, we find that the cells are no
longer all alike or nearly so, but that some are different
from the others in shape, structure and function. The
cells are not all equivalent, the plant is not now a collec-
tion of nearly independent individual parts (cells) ))ut
the whole must be considered as an individual made up
of numerous differentiated parts. It is true that in the
history of every plant there occurs a one-celled stage and
27
28 THE TISSUES OF PLANTS
by the division of this cell the plant originates, but none-
the-less the whole plant is to be considered as a unit and
not as an association of distinct cells.
38. In such higher plants we can distinguish several
types of differentiated cells and can with correctness
speak of tissues. A tissue may be defined as an associa-
tion of similar cells for a common function. In the less
differentiated plants the same tissue will have many
different functions; in the more highly specialized forms
there will be more kinds of tissues each with few^er func-
tions. In the study of tissues we must distinguish
between the so-called ''false" and 'Hrue" tissues. The
former are those that are formed by the subsequent close
association of cells that originated independently of one
another. Thus many separate motile cells (zoospores)
may join themselves to one another in such a way as to
form a definite structure (e.g. Hydrodictyon) or a sort of
tissue may be formed by the growing together of numer-
ous originally separate filaments of cells. On the other
hand a true tissue is formed by successive divisions from
one or a few cells, so that every cell may be said to have
been formed in place. In the false tissues the walls
between adjacent filaments or cells of different origin are
double, without a true middle lamella while in true
tissues the walls are single and the middle lamella is
present (at least at first). It is sometimes impossible to
make a very sharp distinction between these two kinds
of tissues as one method of origin may be combined with
the other. False tissues are found almost exclusively
in the higher fungi and some of the algae while the tissues
of the higher plants are true tissues.
In the following discussion only the more highly
differentiated types of tissues, such as occur in the higher
plants, will be described in their main features while the
IVIERISTEM, AND PARENCHYMA 29
loss difTereiitiiited or more gciKU'alized tissues of the lower
plants will not be considered.
39. Meristem. This is the form of tissue from which
ultimately all the other kinds arise. It is often spoken
of as rudimentary tissue from this fact. It consists of
small, usually rapidly dividing cells (at least during; the
growing season), some of which usually continue as
meristem, while others by enlarging and ceasing their
active division and by other modifications become other
kinds of tissues. Meristem is present in those parts of
the plant where new cells are being formed, i.e. in young
buds, at the apex of growing stems and roots, in the
developing seeds, etc. Meristem cells are usually small
and very thin-walled, and filled with cytoplasm, and
with a nucleus which is large in proportion to the size of
the cell and mostly central in location.
The vacuoles are small or entirely want-
ing. At the growing points of stems and
roots the cells are usually nearly cubical,
in other locations (e.g. cambium) they
may be elongated. If the plant be one ^ « ,, .
. \ . . , , . Fig. 9.— Moristem
with plastids they are present in men- tissue.
stem cells often as a single, very small, hardly distin-
guishable body. Some botanists, however, are of the
opinion that plastids are newly formed in the tissues
developed from the meristem.
40. Parenchyma. This is the chief vegetative tissue
of the higher i)lants and makes up much the larger part
of the living portions of the plant. It is the main nutri-
tive, storage and rei:)roductive tissue. Its cells are
much larger than those of meristem, from which it is
directly derived, but they preserve in general much the
same shape, i.e. they are rounded or polyhedral and usually
not much elongated. The cell walls are thicker than
30 THE TISSUES OF PLANTS
in meristem but are still usually thin, although in certain
modifications, e.g. the parenchyma occurring in wood
and sometimes that in the pith of woody twigs, the walls
may be considerabl}- thickened. In composition the
wall is usually a form of cellulose except where thicken-
ing has begun in which case the walls are often lignified.
A large vacuole occupies the center of the cell and leaves
the cytoplasm as a thin parietal layer (i.e. lining the wall)
although there are often cytoplasmic strands running
across the cell from one side to the other through the
vacuole. The nucleus is generally imbedded in the
parietal cytoplasm and appears relatively small owing
to the great increase in size of the cell in its development
from meristem, unaccompanied by a corresponding
increase in the size of the nucleus. The chloroplasts are
well developed in those parenchyma cells exposed to the
light (except of course in plants devoid of chlorophyll).
Very generally at the angles of contact of three or more
parenchyma cells the middle lamella is ruptured or dis-
solved and the corner of each cell be-
comes rounded off leaving a space
which becomes filled with air, a so-
called intercellular space, these form-
ing a continuous aerating system
throughout the living parts of the
Fig. 10.— Parenchyma plant. lu somo parts of a plant,
as in the pith, the parenchyma cells
die early and the cell contents disappear, being re-
placed by air. Probably this occurs by the absorption
of the protoplasm by the adjacent cells.
Laboratory Studies, (a) For undifferentiated cells examine
the one-celled green slime plants (Protococcus) found as a green
coating on the north side of trees or walls and the manj^-celled
pond scums (such as Spirogyra or Zygnema) or one of the sim-
ple filamentous blue-green algae (as Oscillatoria) which often
LABORATORY STUDIES 31
forms a purplish or brown slimy layer on flower pots in
greenhouses.
(b) For false tissues examine a longitudinal section of the
stalk of a toadstool. Here the longitudinal rows of cells are
distinct filaments grown together into one mass. Similarly
the basal portion of the apothecium of cup-fungi is made up of
false tissue, although here the separate filaments are often
indistinguishable. Some of the algae are also good examples,
e.g. Udotea, Lemanea, Nemalion, etc,
(c) For meristem examine a thin longitudinal section of a
root tip. For this purpose the first strong root from a ger-
minating grain of Indian corn or the j^oung, so-called 'Morace
roots" from near the base of the stem of that plant are good, as
are young roots from onion or h3^acinth bulbs. By staining
lightly with eosin or safranin the nuclei and cytoplasm become
more distinct.
(d) ]\Iake similar longitudinal sections of a very young flower-
or leaf-bud, e.g. lilac or elder, or of the growing tip of asparagus
or of a pumpkin or squash vine and examine the meristem tis-
sue. Compare the cells with those in corresponding locations
in sections made in the older parts of the stem.
(e) For parenchyma cells make thin longitudinal and cross-
sections of a young green stem of Indian corn or of a green shoot
of elder. Excluding the woody and epidermal parts the bulk
of the stem at this stage consists of parenchyma. Treat the
section with iodine solution and then with sulphuric acid. A
blue coloration indicates cellulose.
(f) Make a cross-section of a typical leaf such as apple, lily,
nasturtium, etc. The green cells are parenchyma tissue.
(g) IMake a thin section of the tul^er of potato to show
storage parcncln^ma. Similar parenchyma may be found in
the fruit of an apple or pear, etc.
(h) In thin cross or tangential sections of a living woody twig
will be found the medullary ra3^s. These consist of rather thick-
walled living parenchyma, the walls being more or less lignified
and provided with thin spots (pits) here and there through
which water and food substances can pass from cell to cell.
Stain different sections with iodine and sulphuric acid as a test
for cellulose, and with a five percent aqueous solution of ])hlo-
roglucin and hydrochloric acid as a test for lignified cell walls,
the latter taking a red coloration. Examine in similar manner