Elements of Structural and Systematic Botany

D >> Douglas Houghton Campbell >> Elements of Structural and Systematic Botany

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[14] In a number of plants with showy flowers, _e.g._ violets,
jewel-weed, small, inconspicuous flowers are also formed, which are
self-fertilizing. These inconspicuous flowers are called
"cleistogamous."

At first sight it would appear that most flowers are especially
adapted for self-fertilization; but in fact, although stamens and
pistils are in the same flower, there are usually effective
preventives for avoiding self-fertilization. In a few cases
investigated, it has been found that the pollen from the flower will
not germinate upon its own stigma, and in others it seems to act
injuriously. One of the commonest means of avoiding self-fertilization
is the maturing of stamens and pistils at different times. Usually the
stamens ripen first, discharging the pollen and withering before the
stigma is ready to receive it, _e.g._ willow-herb (Fig. 113, _D_),
campanula (Fig. 123, _A_, _D_), and pea; in the two latter, the pollen
is often shed before the flower opens. Not so frequently the stigmas
mature first, as in the plantain (Fig. 121, _G_).

In many flowers, the stamens, as they ripen, move so as to place
themselves directly before the entrance to the nectary, where they are
necessarily struck by any insect searching for honey; after the pollen
is shed, they move aside or bend downward, and their place is taken by
the pistil, so that an insect which has come from a younger flower
will strike the part of the body previously dusted with pollen against
the stigma, and deposit the pollen upon it. This arrangement is very
beautifully seen in the nasturtium and larkspur (Fig. 99, _J_).

The tubular flowers of the _Sympetalae_ are especially adapted for
pollination by insects with long tongues, like the bees and
butterflies, and in most of these flowers the relative position of the
stamens and pistil is such as to ensure cross-fertilization, which in
the majority of them appears to be absolutely dependent upon insect
aid.

The great orchid family is well known on account of the singular form
and brilliant colors of the flowers which have no equals in these
respects in the whole vegetable kingdom. As might be expected, there
are numerous contrivances for cross-fertilization among them, some of
which are so extraordinary as to be scarcely credible. With few
exceptions the pollen is so placed as to render its removal by insects
necessary. One of the simpler contrivances is readily studied in the
little spring-orchis (Fig. 89) or one of the _Habenarias_ (Fig. 90,
_G_). In the first, the two pollen masses taper below where each is
attached to a viscid disc which is covered by a delicate membrane.
These discs are so placed that when an insect enters the flower and
thrusts its tongue into the spur of the flower, its head is brought
against the membrane covering the discs, rupturing it so as to expose
the disc which adheres firmly to the head or tongue of the insect,
the substance composing the disc hardening like cement on exposure to
the air. As the insect withdraws its tongue, one or both of the pollen
masses are dragged out and carried away. The action of the insect may
be imitated by thrusting a small grass-stalk or some similar body into
the spur of the flower, when on withdrawing it, the two pollen masses
will be removed from the flower. If we now examine these carefully, we
shall see that they change position, being nearly upright at first,
but quickly bending downward and forward (Fig. 89, _D_, ii, iii), so
that on thrusting the stem into another flower the pollen masses
strike against the sticky stigmatic surfaces, and a part of the pollen
is left adhering to them.

The last arrangement that will be mentioned here is one discovered by
Darwin in a number of very widely separated plants, and to which he
gave the name "heterostylism." Examples of this are the primroses
(_Primula_), loosestrife (_Lythrum_), partridge-berry (_Mitchella_),
pickerel-weed (_Pontederia_), (Fig. 84, _I_), and others. In these
there are two, sometimes three, sets of flowers differing very much in
the relative lengths of stamens and pistil, those with long pistils
having short stamens and _vice versa_. When an insect visits a flower
with short stamens, that part is covered with pollen which in the
short-styled (but long-stamened) flower will strike the stigma, as the
pistil in one flower is almost exactly of the length of the stamens in
the other form. In such flowers as have three forms, _e.g._
_Pontederia_, each flower has two different lengths of stamens, both
differing from the style of the same flower. Microscopic examination
has shown that there is great variation in the size of the pollen
spores in these plants, the large pollen from the long stamens being
adapted to the long style of the proper flower.

It will be found that the character of the color of the flower is
related to the insects visiting it. Brilliantly colored flowers are
usually visited by butterflies, bees, and similar day-flying insects.
Flowers opening at night are usually white or pale yellow, colors best
seen at night, and in addition usually are very strongly scented so
as to attract the night-flying moths which usually fertilize them.
Sometimes dull-colored flowers, which frequently have a very offensive
odor, are visited by flies and other carrion-loving insects, which
serve to convey pollen to them.

Occasionally, flowers in themselves inconspicuous are surrounded by
showy leaves or bracts which take the place of the petals of the
showier flowers in attracting insect visitors. The large dogwood
(Fig. 110, _J_), the calla, and Jack-in-the-pulpit (Fig. 86, _A_) are
illustrations of this.

CHAPTER XXI.

HISTOLOGICAL METHODS.

In the more exact investigations of the tissues, it is often necessary
to have recourse to other reagents than those we have used hitherto,
in order to bring out plainly the more obscure points of structure.
This is especially the case in studies in cell division in the higher
plants, where the changes in the dividing nucleus are very
complicated.

For studying these the most favorable examples for ready
demonstration are found in the final division of the pollen spores,
especially of some monocotyledons. An extremely good subject is
offered by the common wild onion (_Allium Canadense_), which flowers
about the last of May. The buds, which are generally partially
replaced by small bulbs, are enclosed in a spathe or sheath which
entirely conceals them. Buds two to three millimetres in length
should be selected, and these opened so as to expose the anthers.
The latter should now be removed to a slide, and carefully crushed
in a drop of dilute acetic acid (one-half acid to one-half
distilled water). This at once fixes the nuclei, and by examining
with a low power, we can determine at once whether or not we have
the right stages. The spore mother cells are recognizable by their
thick transparent walls, and if the desired dividing stages are
present, a drop of staining fluid should be added and allowed to act
for about a minute, the preparation being covered with a cover
glass. After the stain is sufficiently deep, it should be carefully
withdrawn with blotting paper, and pure water run under the cover
glass.

The best stain for acetic acid preparations is, perhaps, gentian
violet. This is an aniline dye readily soluble in water. For our
purpose, however, it is best to make a concentrated, alcoholic
solution from the dry powder, and dilute this as it is wanted. A
drop of the alcoholic solution is diluted with several times its
volume of weak acetic acid (about two parts of distilled water to
one of the acid), and a drop of this mixture added to the
preparation. In this way the nucleus alone is stained and is
rendered very distinct, appearing of a beautiful violet-blue color.

If the preparation is to be kept permanently, the acid must all be
washed out, and dilute glycerine run under the cover glass. The
preparation should then be sealed with Canada balsam or some other
cement, but previously all trace of glycerine must be removed from
the slide and upper surface of the cover glass. It is generally best
to gently wipe the edge of the cover glass with a small brush
moistened with alcohol before applying the cement.

[Illustration: FIG. 127.--_A_, pollen mother cell of the wild onion.
_n_, nucleus. _B-F_, early stages in the division of the nucleus.
_par._ nucleolus; acetic acid, gentian violet, x 350.]

If the spore mother cells are still quite young, we shall find the
nucleus (Fig. 127, _A_, _n_) comparatively small, and presenting a
granular appearance when strongly magnified. These granules, which
appear isolated, are really parts of filaments or segments, which
are closely twisted together, but scarcely visible in the resting
nucleus. On one side of the nucleus may usually be seen a large
nucleolus (called here, from its lateral position, paranucleus), and
the whole nucleus is sharply separated from the surrounding
protoplasm by a thin but evident membrane.

The first indication of the approaching division of the nucleus is
an evident increase in size (_B_), and at the same time the colored
granules become larger, and show more clearly that they are in lines
indicating the form of the segments. These granules next become more
or less confluent, and the segments become very evident, appearing
as deeply stained, much-twisted threads filling the nuclear cavity
(Fig. 127, _C_), and about this time the nucleolus disappears.

The next step is the disappearance of the nuclear membrane so that
the segments lie apparently free in the protoplasm of the cell. They
arrange themselves in a flat plate in the middle of the cell, this
plate appearing, when seen from the side, as a band running across
the middle of the cell. (Fig. 127, _D_, shows this plate as seen
from the side, _E_ seen from above.)

About the time the nuclear plate is complete, delicate lines may be
detected in the protoplasm converging at two points on opposite
sides of the cell, and forming a spindle-shaped figure with the
nuclear plate occupying its equator. This stage (_D_), is known as
the "nuclear spindle." The segments of the nuclear plate next divide
lengthwise into two similar daughter segments (_F_), and these then
separate, one going to each of the new nuclei. This stage is not
always to be met with, as it seems to be rapidly passed over, but
patient search will generally reveal some nuclei in this condition.

[Illustration: FIG. 128.--Later stages of nuclear divisions in the
pollen mother cell of wild onion, x 350. All the figures are seen from
the side, except _B_ ii, which is viewed from the pole.]

Although this is almost impossible to demonstrate, there are
probably as many filaments in the nuclear spindle as there are
segments (in this case about sixteen), and along these the nuclear
segments travel slowly toward the two poles of the spindle
(Fig. 128, _A_, _B_). As the two sets of segments separate, they are
seen to be connected by very numerous, delicate threads, and about
the time the young nuclei reach the poles of the nuclear spindle,
the first trace of the division wall appears in the form of isolated
particles (microsomes), which arise first as thickenings of these
threads in the middle of the cell, and appear in profile as a line
of small granules not at first extending across the cell, but later,
reaching completely across it (Fig. 128, _C_, _E_). These granules
constitute the young cell wall or "cell plate," and finally coalesce
to form a continuous membrane (Fig. 128, _F_).

The two daughter nuclei pass through the same changes, but in
reverse order that we saw in the mother nucleus previous to the
formation of the nuclear plate, and by the time the partition wall
is complete the nuclei have practically the same structure as the
first stages we examined (Fig. 128, _F_).[15]

[15] The division is repeated in the same way in each cell so that
ultimately four pollen spores are formed from each of the original
mother cells.

This complicated process of nuclear division is known technically as
"karyokinesis," and is found throughout the higher animals as well
as plants.

The simple method of fixing and staining, just described, while giving
excellent results in many cases, is not always applicable, nor as a
rule are the permanent preparations so made satisfactory. For
permanent preparations, strong alcohol (for very delicate tissues,
absolute alcohol, when procurable, is best) is the most convenient
fixing agent, and generally very satisfactory. Specimens may be put
directly into the alcohol, and allowed to stay two or three days, or
indefinitely if not wanted immediately. When alcohol does not give
good results, specimens fixed with chromic or picric acid may
generally be used, and there are other fixing agents which will not be
described here, as they will hardly be used by any except the
professional botanist. Chromic acid is best used in a watery solution
(five per cent chromic acid, ninety-five per cent distilled water).
For most purposes a one per cent solution is best; in this the objects
remain from three or four to twenty-four hours, depending on size, but
are not injured by remaining longer. Picric acid is used as a
saturated solution in distilled water, and the specimen may remain for
about the same length of time as in the chromic acid. After the
specimen is properly fixed it must be thoroughly washed in several
waters, allowing it to remain in the last for twenty-four hours or
more until all trace of the acid has been removed, otherwise there is
usually difficulty in staining.

As staining agents many colors are used. The most useful are
haematoxylin, carmine, and various aniline colors, among which may be
mentioned, besides gentian violet, safranine, Bismarck brown, methyl
violet. Haematoxylin and carmine are prepared in various ways, but are
best purchased ready for use, all dealers in microscopic supplies
having them in stock. The aniline colors may be used either dissolved
in alcohol or water, and with all, the best stain, especially of the
nucleus, is obtained by using a very dilute, watery solution, and
allowing the sections to remain for twenty-four hours or so in the
staining mixture.

Haematoxylin and carmine preparations may be mounted either in
glycerine or balsam. (Canada balsam dissolved in chloroform is the
ordinary mounting medium.) In using glycerine it is sometimes
necessary to add the glycerine gradually, allowing the water to slowly
evaporate, as otherwise the specimens will sometimes collapse owing to
the too rapid extraction of the water from the cells. Aniline colors,
as a rule, will not keep in glycerine, the color spreading and finally
fading entirely, so that with most of them the specimens must be
mounted in balsam.

Glycerine mounts must be closed, which may be done with Canada balsam
as already described. The balsam is best kept in a wide-mouthed
bottle, specially made for the purpose, which has a glass cap covering
the neck, and contains a glass rod for applying the balsam.

Before mounting in balsam, the specimen must be completely freed from
water by means of absolute alcohol. (Sometimes care must be taken to
bring it gradually into the alcohol to avoid collapsing.[16]) If an
aniline stain has been used, it will not do to let it stay more than a
minute or so in the alcohol, as the latter quickly extracts the stain.
After dehydrating, the specimen should be placed on a clean slide in a
drop of clove oil (bergamot or origanum oil is equally good), which
renders it perfectly transparent, when a drop of balsam should be
dropped upon it, and a perfectly clean cover glass placed over the
preparation. The chloroform in which the balsam is dissolved will soon
evaporate, leaving the object embedded in a transparent film of balsam
between the slide and cover glass. No further treatment is necessary.
For the finer details of nuclear division or similar studies, balsam
mounts are usually preferable.

[16] For gradual dehydrating, the specimens may be placed
successively in 30 per cent, 50 per cent, 70 per cent, 90 per cent,
and absolute alcohol.

It is sometimes found necessary in sectioning very small and delicate
organs to embed them in some firm substance which will permit
sectioning, but these processes are too difficult and complicated to
be described here.

* * * * *

The following books of reference may be recommended. This list is, of
course, not exhaustive, but includes those works which will probably
be of most value to the general student.

1. GOEBEL. Outlines of Morphology and Classification.

2. SACHS. Physiology of Plants.

3. DE BARY. Comparative Anatomy of Ferns and Phanerogams.

4. DE BARY. Morphology and Biology of Fungi, Mycetozoa, and Bacteria.

These four works are translations from the German, and take the
place of Sachs's Text-book of Botany, a very admirable work
published first about twenty years ago, and now somewhat antiquated.
Together they constitute a fairly exhaustive treatise on general
botany.--New York, McMillan & Co.

5. GRAY. Structural Botany.--New York, Ivison & Co.

6. GOODALE. Physiological Botany.--New York, Ivison & Co.

These two books cover somewhat the same ground as 1 and 2, but are
much less exhaustive.

5. STRASBURGER. Das Botanische Practicum.--Jena.

Where the student reads German, the original is to be preferred, as
it is much more complete than the translations, which are made from
an abridgment of the original work. This book and the next (7 and 8)
are laboratory manuals, and are largely devoted to methods of work.

7. ARTHUR, BARNES, and COULTER. Plant Dissection.--Holt & Co., New
York.

8. WHITMAN. Methods in Microscopic Anatomy and Embryology.--Casino
& Co., Boston.

For identifying plants the following books may be mentioned:--

Green algae (exclusive of desmids, but including _Cyanophyceae_ and
_Volvocineae_).

WOLLE. Fresh-water Algae of the United States.--Bethlehem, Penn.

Desmids. WOLLE. Desmids of the United States.--Bethlehem, Penn.

The red and brown algae are partially described in FARLOW'S New England
Algae. Report of United States Fish Commission, 1879.--Washington.

The _Characeae_ are being described by Dr. F. F. ALLEN of New York. The
first part has appeared.

The literature of the fungi is much scattered. FARLOW and TRELEASE
have prepared a careful index of the American literature on the
subject.

Mosses. LESQUEREUX and JAMES. Mosses of North America.--Boston, Casino
& Co.

BARNES. Key to the Genera of Mosses.--Bull. Purdue School of Science,
1886.

Pteridophytes. UNDERWOOD. Our Native Ferns and their Allies.--Holt
& Co., New York.

Spermaphytes. GRAY. Manual of the Botany of the Northern United
States. 6th edition, 1890. This also includes the ferns, and the
liverworts.--New York, Ivison & Co.

COULTER. Botany of the Rocky Mountains.--New York, Ivison & Co.

CHAPMAN. Flora of the Southern United States.--New York, 1883.

WATSON. Botany of California.

INDEX.

_Acacia_, 209.

_Acer_, _-aceae_. See "Maple."

Acetic acid, 3, 59, 98, 138, 230.

_Achimenes_, 218.

_Acorus_. See "Sweet-flag."

Actinomorphic, 213.

Adder-tongue, 116; Fig. 70. See also "_Erythronium_."

_Adiantum_. See "Maiden-hair."

_Adlumia_. See "Mountain-fringe."

_AEsculinae_, 199.

_AEsculus_. See "Buckeye," "Horse-chestnut."

_Aggregatae_, 222.

Alcohol, 5, 31, 55, 83, 230, 233.

Algae, 4, 21.
green, 21.
red, 21, 49.
brown, 21, 41.

Alga-fungi. See "_Phycomycetes_."

_Alisma_, _-ceae_. See "Water-plantain."

_Allium_. See "Wild onion."

Amaranth, 185.

_Amarantus_, _-aceae_. See "Amaranth."

_Amoeba_, 7; Fig. 2.

_Ampelidae_. See "Vine."

_Ampelopsis_. See "Virginia creeper."

Anatomy, 3.
gross, Implements for study of, 3.
minute, Implements for study of, 3, 4.

Anatropous, 151.

_Andreaeaceae_, 99, 100.

Androecium, 148.

_Andromeda_, 211.

_Anemone_, 185.

_Angiocarpae_, 84.

Angiosperm, 129, 143, 145.

Aniline colors, 233.

_Anisocarpae_, 210, 213.

_Anonaceae_. See "Custard-apple."

Anther, 148, 175, 179.

Antheridium, 27, 36, 39, 45, 51, 59, 68, 89, 96, 106, 122.

_Anthoceros_, _Anthoceroteae_, 91; Fig. 57.

_Aphanocyclae_, 185, 196.

_Aplectrum_, 167; Fig. 90.

_Apocynum_, _-aceae_. See "Dog-bane."

_Apostasieae_, 164.

Apple, 145, 171, 206; Fig. 114.

Apricot, 207.

_Aquilegia_. See "Columbine."

_Aralia_, _-aceae_. See "Spikenard."

Archegonium, 89, 97, 105, 122, 133, 140, 144.

Archicarp, 138, 145.

_Arcyria_, 13; Fig. 5.

_Arethusa_, _Arethuseae_, 166; Fig. 90.

_Argemone_, 191.

Aril, 189.

_Arisaema_, 78, 157; Fig. 86.

_Aristolochia_, _-aceae_, 224.

Aroid, _Aroideae_, 157.

Arrow-grass, 167.

Arrowhead, 167; Fig. 91.

Arrowroot, 163.

_Asarum_. See "Wild ginger."

_Asclepias_, _-daceae_. See "Milk-weed."

_Ascobolus_, 71-73; Fig. 43.
culture of, 71.
spore fruit, 71.
archicarp, 71.
spore sacs, 72.

_Ascomycetes_, 65, 66.

Ascospore, 66.

Ascus, 66, 69.

Ash, 218; Fig. 122.

_Asimina_. See "Papaw."

_Aspidium_, Fig. 70.

_Asplenium_, 104; Fig. 70.

Aster, 224.

_Atropa_. See "Deadly nightshade."

Axil, 174.

Azalea, 210; Fig. 116.

_Azolla_, 117; Fig. 71.

Bacteria, 15, 17, 19; Fig. 8.

Balsam, _Balsamineae_, 198.

Bamboo, 162.

_Bambusa_. See "Bamboo."

Banana, 163.

Barberry, 17, 187; Fig. 101.

Bark. See "Cortex."

_Basidiomycetes_, 77.

Basidium, 77, 80, 83.

Basswood, 195; Fig. 106.

Bast. See "Phloem."

_Batatas_. See "Sweet-potato."

_Batrachospermum_, 53; Fig. 31.

Bean, 207, 208.

Bear-grass. See "_Yucca_."

Bee, 227, 228.

Beech, 183.

Beech-drops, 218.

Beet, 184.

Beggar's-ticks, 215.

Begonia, 3, 205.

Bell-flower, 220, 226; Fig. 123.

Bellwort, 156.

_Berberis_, _-ideae_. See "Barberry."

Bergamot oil, 234.

Berry, 145, 156.

_Betulaceae_, 183.

_Bicornes_, 210.

_Bignonia_, _-aceae_, 218.

Biology, 2.

Birch, 183.

Bird's-nest fungus. See "_Cyathus_."

Bishop's cap, 202; Fig. 111.

Bismarck brown, 233.

Bitter-sweet, 199; Fig. 109.

Black alder, 199.

Blackberry, 207.

Black fungi. See "_Pyrenomycetes_."

Bladder-nut, 199; Fig. 108.

Bladder-weed, 33, 217; Fig. 120.

Bleeding-heart. See "_Dicentra_."

Blood-root, 191; Fig. 103.

Blue-eyed grass, 156.

Blue-flag. See "_Iris_."

Blue-green slime, 15.

Blue valerian. See "_Polemonium_."

Borage, 215.

_Borragineae_. See "Borage."

Bordered pits, 138.

Botany defined, 2.
systematic, 3.

_Botrychium_. See "Grape fern."

Box, 201.

Bract, 199, 222, 229.

_Brasenia_. See "Water-shield."

Breathing pore, 91, 99, 113, 130, 147, 150, 177.

_Bromeliaceae_, 156.

Bryophyte, 86.

Buck-bean, 218.

Buckeye, 171, 199.

Buckthorn, 199.

Buckwheat, 184.

Budding, 64.

_Bulbochaete_, 28; Fig. 16.

Bulb, 146, 153, 172.

Bulrush, 161; Fig. 87.

Bundle-sheath, 110, 176.

Burning-bush. See "Spindle-tree."

Bur-reed, 159; Fig. 86.

Buttercup, 181, 185; Fig. 99.

Butterfly, 227, 228.

Button-bush, 223.

Buttonwood. See "Sycamore."

_Buxus_, _Buxaceae_. See "Box."

Cabbage, 192.

_Cabombeae_, 190.

Cactus, _Cactaceae_, 203; Fig. 112.

_Caesalpineae_, 210.

Calcium, 2.

Calla, 157, 229.

_Callithamnion_, 50-52; Fig. 29.
general structure, 51.
tetraspores, 51.
procarp, 51.
antheridium, 51.
spores, 52.

_Callitriche_, _-chaceae_. See "Water starwort."

_Calluna_. See "Heath."

_Calopogon_, 166; Fig. 91.

_Calycanthus_, _-aceae_, 187; Fig. 100.

_Calycereae_, 223.

_Calyciflorae_, 200.

Calyx, 174, 182.

Cambium, 137-138, 175.

_Campanula_. See "Bell-flower."

_Campanulaceae_, 220.

_Campanulinae_, 220.

Canada balsam, 230-234.

Canada thistle, 224; Fig. 125.

_Canna_, _-aceae_, 162, 163; Fig. 88.

Caper family, 194.

_Capparis_, _-ideae_. See "Caper."

_Caprifoliaceae_, 223.

_Capsella_. See "Shepherd's-purse."

Caraway, 202.

Carbon, 2, 95.

Carbon-dioxides, 95.

Cardinal-flower. See "Lobelia."

_Carex_, 161; Fig. 87.

Carmine, 25, 233.

Carnation, 185.

Carpel, 148, 154, 175, 179.

Carpophyll. See "Carpel."

Carpospore, 51-53.

Carrot, 202.

_Caryophylleae_. See "Pink."

_Caryophyllus_. See "Clove."

_Castalia_, 189.

Castor-bean, 200.

Catalpa, 218.

Cat-brier, 154.

Catkin, 181.

Catnip, 215.

Cat-tail, 159.

Cedar apple, Cedar rust. See "_Gymnosporangium_."

_Celastraceae_, 199.

_Celastrus_. See "Bitter-sweet."

Celery, 3.

Cell, 6.
apical, 38, 96, 105, 115.
division, 23, 31, 229.
row, 8; Fig. 3.
mass, 8; Fig. 4.
sap, 6, 151.

Cellulose, 3.

Centaury, 219.

_Centrospermae_, 183.

_Cephalanthus_. See "Button-bush."

_Cerastium_. See "Chick-weed."

_Ceratophyllum_. See "Horned pond-weed."

_Cercis_. See "Red-bud."

_Chamaerops_. See "Palmetto."

_Chara_, 38-40; Fig. 23.
general structure, 38.
method of growth, 39.
cortex, 39.
non-sexual reproduction, 39.
ooegonium, 39.
antheridium, 39, 40.
spermatozoids, 40.
germination, 40.

_Characeae_, 21, 37, 40.

_Chareae_, 40.

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