drates. These are composed of carbon, hydrogen and
oxygen in the proportion, usually, of six parts of carbon,
ten of hydrogen and five of oxygen. In many of the
fungi and some other plants the cell wall is composed of
a form of chitin, containing nitrogen in addition to the
substances mentioned. This has been called fungus
cellulose, although not related to cellulose chemically.
In the walls of older cells there are frequently deposited
various other substances such as silica in the diatoms
and in the epidermal cells of joint rushes and grasses,
suberin and cutin in the walls of cork and epidermal cells,
respectively, hadromal, or perhaps vanillin and conif-
erin in wood cells, etc., these being in part the so-called
''Ugnin" of earlier botanical works. Aside from cellu-
lose the chief constituent of cell walls is pectose, chemi-
cally very similar to it and frequently mixed with it.
Under the influence of certain not well understood
6 PROTOPLASM AND PLANT CELLS
conditions the cellulose or pectose may become changed
into gums, e.g. gum arable, cherry gum, slime of flax-
seed, etc.
10. The cell wall when first formed is very thin.
Growth occurs either by apposition (deposition of cell
wall substance on the inner surface of the wall) in which
case the wall becomes thicker and may or may not
appear layered, or by intussusception (the deposition of
new material among the particles of the old), in which
case the wall becomes larger as well as often thicker.
The first laj^er formed is the thin middle lamella. Upon
this is deposited, on either side, a thicker layer of some-
what different composition, the secondary lamella. A
tertiary lamella is sometimes formed also. These
different layers are usually of somewhat different chemi-
cal composition. Thus the middle lamella is often com-
posed of calcium pectate or some other pectose compound
while the secondary lamellae are cellulose or a mixture
of cellulose with other substances. When present, the
tertiary lamella is usually nearly pure cellulose.
11. The walls between adjacent living cells are quite
generally perforated by very minute pores through which
delicate fibrils of cytoplasm extend from one cell to the
other, apparently thus binding all the
living cells of the plant together into one
more or less coordinated unit.
12. The thickening of the cell wall is
not always uniform. Indeed, except in
Fig. 2.— Thick- Comparatively thin-walled cells thinner
areas or spots are almost always left be
tween the more thickened parts. These thickenings may
be ridges which are in the shape of rings, spirals or reticu-
lations or may occupy so much of the surface that the
unthickened parts appear as pits. Usually these thick-
CHARACTERISTICS OF CELLS 7
enings are on the inner surface of the cell wall, but in
many spores (e.g. pollen grains or spores of ferns or fungi)
they are external. This is also the case in some of
the lower, one-celled plants such as desmids. The
thickenings have various functions, such as strengthen-
ing the wall, providing means for transportation (in the
case of spores and pollen grains which sometimes depend
upon animals for their dispersal, the rough projections
enabling them to cling to the animal), etc.
13. After attaining their full differentiation most of
the cells of the higher plants (at least of the woody
plants) die, their cell walls remaining to make up the
bulk of the plant body. We usually continue to speak
of such dead, empty cell walls as cells, although the
essential parts, the cytoplasm and nucleus, may have
disappeared long ago.
14. Cells vary greatly in size, those of some of the
bacteria being less than half a micron (i.e. less than one-
fifty-thousandth of an inch) in diameter, wdiile the egg
cell of Zamia may have a thickness of over a millimeter
and a length of 3 mm. (i.e. a volume over twenty billion
times as great), the egg cell of Dioon being even larger.
Some fiber cells have a length of many centimeters, e.g.
bast fibers of ramie {Boehmeria nivea).
15. In some of the lower aquatic plants occur repro-
ductive cells with no cell walls (e.g. zoospores, tetra-
spores, etc.). These cells are frequently motile by means
of protoplasmic processes called cilia or flagella. Such
cells in many cases settle down and, becoming attached
to something, form a cell wall before proceeding further
in their development. Even in the higher plants the egg
and sperm cells are naked.
16. Typical cells have but a single nucleus. In certain
stages of the life history of some groups of plants the
8 protoplas:m and plant cells
cells are binucleate while they are uninucleate in the
remaining stages. In some groups of plants, however,
we find that, enclosed in an outer cell wall, there is a
mass of cytoplasm containing many nuclei. Such a
structure is called a coenocyte. It is frequently re-
garded as consisting of as many cells as nuclei are present,
not separated, however, by partition w^alls. Perhaps it
may better be considered as a sort of compound cell as
the nuclei do not seem to control definite masses of cy-
toplasm. In some coenocytes of the seaweed Griffithsia
over 4,000 nuclei are present, while in the enormous
coenocyte of Caulerpa, likewise a seaweed, which often
attains a length of several decimeters, the number of
nuclei is vastly greater. Coenocytes are mostly re-
stricted to certain groups of lower plants, but cells of
coenocytic nature may occur even in the higher plants.
17. In shape cells are very variable. Usually we find
that free-living cells approach the spherical shape al-
though they are often elongated somewhat. Cells
united to other cells are usually flattened on the sides
where they are in contact. When surrounded by cells
at all sides cells are usually more or less regular, several
to many-sided polyhedra. Some cells are cylindrical
while often we have fiber or spindle shaped cells. Some
cells are lobed or branched.
Laboratory Studies. It is assumed that the attempt will
not be made to use this book without endeavoring to carry
out in the laboratory all or at least a selection of the laboratory
exercises suggested here and there in connection with the
various topics. So far as possible the suggested exercises
have been made simple enough for the student to undertake
himself, depending as little as possible upon specimens prepared
or experiments set up by the teacher. It is absolutely essential
that each student have the use of a good compound micro-
scope, and that he possess the proper tools for making sections,
LABORjVTORY STUDIES 9
etc., as well as a few siini)le reagents such as alcohol, iodinc-
potassium-iodide solution, potash solution, etc. The measure-
ments used throughout this book arc metric; 1 cm. = 0.394 in.
1 mm. = about 1/25 inch, 1 micron (written At)= 0.001 mm.
(i.e. about one-twcnt3''-five-thousandth of an inch).
(a) ]\Iake a thin longitudinal section of the tip of a large
root of Indian corn or hyacinth or any other plant with stout
roots, or of the growing point of a herbaceous stem, and
mount in water and examine under the microscope. The
small cells near the tip will be found to be full of protoplasm.
The following tests should be made on different sections: (1)
Add strong iodine solution; this turns the protoplasm brown
or yellowish brown. (2) Test with a drop or two of Millon's
reagent (dissolve a small amount of mercury in an equal weight
of strong nitric acid, and dilute with an equal amount of
distilled water. Use fresh): the protoplasm is turned bright
yellow. (3) Mount a section in strong sugar solution and
after a few moments add a drop of fairly strong sulphuric
acid: the protoplasm is stained red or pink. (4) Treat a
section with nitric acid and then with strong potash: the yellow
color of the protoplasm shows the so-called xanthoprotein
reaction.
(6) Repeat these tests with raw white of egg, which consists
of proteins. Note that the results are the same. For the
sulphuric-acid-sugar test it is more satisfactory to mix the egg
white with a strong sugar solution in a test tube, rolhng the
latter so that the sides are moistened with the mixture. Now
very carefully run a small drop of concentrated sul])huric acid
down the side of the tube. This browns the solution where
it comes in contact in most concentrated form but at the edge
of its path and at its point of entrance into the mixture tlie
red coloration is shown beautifull}^
(c) To study the motion of cytoplasm make a cross or
longitudinal section of a stem (the upper, younger portion) of
Petunia or tomato without injuring the hairs. JMount in
water and examine a cell of a hair. The cytoplasm will
usually be found to be streaming. Note that the streams seem
frequently to center upon the nucleus. Note the effect upon
the motion of placing the slide on a jiiece of ice. Warm it up
again to a temperature of about 30° to 35° C. and note the
10 PROTOPLASM AND PLANT CELLS
results. Heat to 55° to 60° C. Now cool to about 30°.
Examine again.
{(I) On similar specimens test the effect upon motion of
iodine solution, alcohol, glycerine, etc.
(e) Various types of proto])lasmic motion may be found in
the long cells of the young silk of Indian corn, in the cells of
the leaves of water weed (Philotria), the cells, especially those
near the ends of the shoots, of Chara or Nitella, etc.
(/) To observe the different parts of a cell study again the
stem hairs of Petunia. Note nucleus, nucleolus cytoplasm,
vacuoles, cell wall. Cells from the leaf of a moss may also be
used for this purpose.
(g) Bring into the laboratory some growing LTlothrix,
Cladophora, Stigeoclonium or other zoospore-producing algae,
and place in fresh water near the window. In a few hours one
can often find myriads of zoospores. Examine these for cells
lacking walls and provided with motile organs (flagella).
(h) Make a thin cross-section of a herbaceous stem. Treat
with iodine solution and then with somewhat diluted sulphuric
acid. Cellulose walls are turned blue, cutinized and lignificd
(wood) walls, yellowish brown. Stain another section with
anilin-water safranin. This stains cutin walls yellowish and
lignin walls bluish.
(i) Examine a thread of green felt (Vaucheria) or a vegeta-
tive thread of bread mold (Mucor) for a plant of coenocytic
structure. Note the lack of cross walls. The numerous
minute nuclei are not visible without staining.
ij) The stone cells making up the shells of various nuts are
good objects to show the deposition of the cell wall in layers,
i.e. by apposition. With a pocket knife cut as thin a section as
possible, and place it in water containing a httle potash. At
the edges may be found areas thin enough for examination.
Here and there in the plainly layered cell wall will be found
pits, i.e. thin places left when the rest of the wall thickened.
18. Plastids. Three kinds of plastids occur in plants.
They all agree in general structure in that they are denser
bodies of protoplasm imbedded in the cytoplasm. They
may have many shapes but are more frequently round or
elliptical in outline. So far as is certainly known new
PLASTID8 11
plastids are formed only from the division of old plastids
into two parts. They are difficultly visible in some plant
cells, e.g. in the small rapidly dividing meristem cells at
the growing points of a plant, and are entirely lacking in
some great groups of plants, viz. the bacteria and fungi.
19. Chloroplasts are plastids containing chlorophyll.
Ordinarily they are green, from the color of the chloro-
phyll itself, but in some groups of plants the green color
is masked by the presence of other pigments in the chloro-
plasts in addition to the chlorophyll. Thus
in the Red Seaweeds (Rhodophyceae) the
chloroplasts are usually red, in the Brown
Algae (Phaeophyceae) they are brown, in
some ]\Iyxophyceae the chloroplasts are
bluish green, etc. Chlorophyll proper is
a bluish green, apparently somewhat oily p^^ 3— piistida
substance, probably contained in inter- (cMoropiasts) in a
stices of the chloroplast. It is soluble
in alcohol, by means of which it can be removed, leav-
ing the chloroplast colorless. In addition to chlorophjdl
most chloroplasts contain an orange yellow pigment, to
which the name xanthophjdl is often applied. It ap-
pears to be a form of carotin. The mixture of these
two gives the grass-green color to the chloroplast. With
rare exceptions chlorophyll is not produced in the ab-
sence of light. It usually disappears in prolonged dark-
ness, leaving the chloroplast stained yellow with xantho-
phyll or colorless. In many of the lower plants the
chloroplasts are of various shapes, often being star-,
band-, plate-, or even net-shaped. In the higher plants
they are mostly more or less disk shaped. In some of
the liverworts and many of the algae they contain one
or more highly refractive bodies, called pyrenoids, which
are probably crystals of some albuminous substance.
12 PROTOPLAS:\r AND PLANT CELLS
20. Leucoplasts are colorless plastids occurring in the
parts of the plant not exposed to light. When exposed
to light they usually produce chlorophyll and become
green, showing that they are essentially the same as the
chloroplasts. They are abundant in parts of the plant
where starch is being stored up.
21. Chromoplasts are found in the cells of many
flowers and fruits and other colored parts of plants.
They are small, round or angular or needle shaped
plastids, mostly red or yellow in color. They contain
carotin or other coloring matters but no chlorophyll.
In many cases they are directly developed from chloro-
plasts by the loss of chlorophyll and the development of
some other pigment.
Laboratory Studies. — (a) Mount a leaf of moss and examine
for chloro})Iasts.
(6) Soak a few moss leaves in strong alcohol for twenty-four
hours and note the decoloration of the chloroplasts.
(c) Examine Sj^irogyra for spiral, ribbon-shaped, or Zygnema
for star-shaped chloroplasts.
(^/) Soak a handful of leaves in alcohol for several hours. If
the flask containing the alcohol and leaves be placed in hot
water the extraction of the chlorophyll will progress more
rapidly. Note the green color of the extract. Add a little
gasoline or benzine (not benzene, i.e. benzol) to the alcoholic
solution and shake thoroughly and then let it stand until the
alcohol and gasoline separate. The chlorophyll will be found
now in the gasoline, the carotin remaining in the alcohol.
(e) Examine the cells of various fungi, e.g. toadstools,
puf'fballs, molds, etc., or of a parasitic flowering plant, e.g.
dodder (Cuscuta), and note the absence of chloroplasts.
(/) Sprout a potato in darkness. Make a section of its stem
and compare with a similar section of the stem of a potato
grown in light. Note the leucoplasts in the former and the
chloroplasts in the latter. Similarly compare the stomatal
guard cells of the epidermis of green and l)lanched celery.
{g) Examine the cells of a carrot root for chromoplasts
CELL INCLUSIONS
13
stained with carotin. Examine also the red cells of a ripe
tomato or the yellow cells of a petal of nasturtium (Tropaeo-
lum) or the cells of rose hips.
22. Cell Inclusions. Within many cells are often
found bodies not living and not an essential part of the
cell but which have been produced by the cell itself.
They may be temporary or permanent. They may lie
in the cytoplasm, in the vacuoles or in the plastids.
Such bodies are called cell inclusions. The most fre-
quent cell inclusions are starch, aleuron, crystals and
sometimes drops of fat or oil.
23. Starch. In the green cells of many plants there
are produced in the chloroplasts on exposure to light
small pearly white grains of starch.
These are usually transformed into
sugar during the night and used by the
plant for food or transported to some
other part such as root, tuber or seed,
where the sugar may be again con-
verted to starch, in the leucoplasts, to
Fig
Starch
remain until needed by the plant for f /eC/onUlmiif.' ^ ^"'^
food. Whereas in the green cells of
a leaf the starch does not ordinarilj^ accumulate in great
quantities, the storage cells of a plant become so packed
with it sometimes that little else can be seen.
Starch is a carbohj^drate and is closely related chemi-
cally to cellulose and to the sugars. It is composed of
carbon, hydrogen and oxygen in the proportions indi-
cated by the formula (C6Hio05)n, in which ''n" is a
fairly high Init not exactly ascertained amount. By the
action of certain organic substances produced by the cell
and called enzymes, or of some of the acids and heat, it
can l)e converted into some forms of sugar.
Starch grains frequently show a concentric structure,
14 PROTOPLASM AND PLANT CELLS
due apparently to the successive deposition of denser and
less dense la3'ers. At first the grains are entirely en-
closed by the plastid but as they increase in size they
become excentrically located and seem eventually to
burst out of the plastid at one side. In the chloroplasts
containing pyrenoids the starch grains are mostly pro-
duced in intimate connection with the latter.
24. Aleuron. In the dry seeds of many plants there
may be found, sometimes in a definite layer of cells,
sometimes scattered throughout the cells of the seed,
small rounded or frequently angular granules of a protein
substance called aleuron. This is stored up in the cells
as food for the young seedling. These aleuron grains are
formed in small vacuoles in the cytoplasm, the aleuron
being in solution at first but appearing as granules or
even crystalloids as the seed loses its moisture in the
process of ripening. As the seed absorbs water prepara-
tory to germinating the aleuron goes into solution again
and is used up for food. Aleuron is frequently found in
cells containing other stored up food matter such as
starch or oil. It was formerly supposed to be a dry
stage of protoplasm but is now recognized as one of the
highly complex food substances out of which protoplasm
can be formed by the cell.
25. Oils or Fats. Many plants provide for the use of
the young seedling a supply of fat instead of starch.
This is usually present in the cell as very minute drops,
in fact almost as an emulsion throughout the cytoplasm.
Sometimes the oil droplets are of considerable size, in
very oily seeds often filling all the interstices of the cyto-
plasm. Usually these fats are liquid but in some plants
they are semisolids of the consistency of butter. They
are mostly true fats, similar to those found in animals,
CRYSTALS 15
but in some plants cells are found which contain so-called
''ethereal oils," which are not true fats.
26. Crystals. In many plants may be found cells
containing crystals. These may be cubical, prismatic,
regular or irregular polyhedrons, needles, compound
crystals, etc. Sometimes the cells containing them are
unchanged but often they are enlarged or of special
shape. This is especially the case with the needle-
shaped crystals which are called raphids
and occur in large bundles in the cen-
tral vacuole of rather large, thin-walled
cells. The crystals seem to be formed
by the cytoplasm, in which they occa-
sionally lie, or more frequently in special
small vacuoles in the latter. Eventu- pound." and needTe^
,, , r 1 • , 'XT, shaped crystals.
ally they are found m most cases m the
central vacuole in which some of them may have had
their origin.
27. Crystals in most plants are composed of calcium
oxalate. In some plants calcium carbonate crystals
occur, while crystals of still different composition are
occasionally found. The purpose of crystals is not clear
in all cases but in many cases they are probably the
product of the combination of waste substances set free
in the course of some of the important chemical pro-
cesses of which the cell is constantly the seat.
Laboratory Studies, (a) Make a thin section of a potato
tuber. Mount in water. Note the large, thin-walled cells
packed with numerous ovoid, concentrically marked starch
grains. Treat with iodine solution. The starch grains become
blue or purple. In very young tubers, where the starch grains
are not so large nor so numerous, they may be seen to be
enclosed in leucoplasts.
(b) Study the different types of starch grains in corn, wheat,
rice, oats, etc.
16 PROTOPLASM AND PLAXT CELLS
(c) Place a dish of water containing Spirogyra in the light
for some hours and then examine a few filaments. In the
spirally wound chloroi)lasts, around the pyrenoids will be
found masses of starch which become more evident on staining
with iodine.
{(}) Make thin sections through various leaves that have been
exposed to the light for some time, staining with iodine. In
some of these minute grains of starch will be found in the
chloroplasts.
(e) Make longitudinal sections of ripened apple twigs, in the
fall or winter especially, and note the starch stored in the
rather thick-walled cells of the pith.
(/) IMount in strong alcohol or glycerine a thin section of a
pea or bean. In addition to starch grains the cells will be
found to contain very numerous fine granules. Stain with
iodine. These small aleuron granules will be stained brown
and the starch blue. To another section apply one of the
tests for proteins given on p. 9. Mount another section in
water and note the effect on the aleuron. Examine cotyle-
dons of germinated peas and beans for presence or absence of
aleuron.
(g) Examine a cross-section of a wlicat grain. The aleuron
will be found in a layer of cells outside of the starch-containing
cells. This laj^er is largely removed with the bran in the
process of making flour.
(h) Make a thin section of a seed of the castor oil plant
(Ricinus). Mount without adding water or any other
reagent. Large aleuron grains will be seen, each containing an
angular protein crystal and a spherical, so-called "globoid," of
inorganic nature. Add a little water and some of the oil will
escape and appear at the edges of the section as large drops.
(i) Examine various oily seeds such as cotton, flax, peanut,
or an oily fruit such as the avocado (Persea gratissima) or olive.
In the cells w^ill be found varying amounts of oil. By treating
the sections with 1 per cent, solution of osmic acid or with
alkannin solution the oil will be stained respectively black or
red.
(j) Make a thin longitudinal section of the stem of spider-
wort (Tradescantia) and mount in water. Certain thin-
walled cells will be found containing bundles of needle-shaped
crystals (raphids). Many of these will be torn out of position
CELL SAP
17
and scattered throup;liout the si)cciinen. These crj'stals are
composed of calcium oxalate. Add a little hydrochloric acid
and they will dissolve without effervescence.
(k) Similar crystals may be found in many other plants,
e.g. Lidian turnip (Arisaema), evening primrose (Oenothera),
fuchsia, garden balsam (Impatiens), garden rhulmrb, etc.
(/) For crystals of other types examine sections of prickly
pear (Opuntia), young basswood twigs, scales of onion, stem of
lamb's quarters (Chenopodium), petiole of beet, etc. These
are also composed of calcium oxalate.
(m) Examine a thin cross-section of the leaf of the rubber
plant (Ficus elastica). In some of the modified epidermal
cells will be found peculiar stalked crystalline bodies of calcium
carbonate deposited upon a cellulose core which hangs down
into the cell cavity from the outer jiortion of the cell wall.
Treat the section with, hydrochloric acid. The cystolith, as it
is called, dissolves with the evolution of CO2, leaving the cellu-
lose core, thus distinguishing it from calcium oxalate, which
dissolves without effervescence.
28. Cell Sap. The cytoplasm of a cell usually contains
a large amount of water imbibed by it but not really a
part of it. Water is also found fre-
quently in drops (vacuoles) within
the cell. This is the cell sap. It
holds in solution the various soluble
substances absorbed by the plant as
well as those manufactured by the
cell itself. It makes up by far the
greater part of the bulk of the contents
of the average cell. Among the sub-
stances dissolved in the cell sap, in
addition to the mineral matters absorl^ed by the plant
from the soil water, are many sorts of organic compounds
produced by the cytoplasm. The most important of
these are the various sugars and organic acids. The
commonest of the sugars are saccharose or cane sugar
Fu
-Large vacuoles.
18 PROTOPLAS:^! AND PLANT CELLS
(C12H22O11), glucose or grape sugar (C6H12O6), fructose