CHAPTER III

HOW PLANTS PRODUCE THEIR YOUNG

There is perhaps no device of nature that more perfectly accomplishes

its purpose than the one with which all living things are endowed--the

instinct for the renewal of life. In man the dawn of the mating instinct

has ever been the theme of poets, and some of its manifestations are the

despair of ascetics. Through it some of the noblest of man’s emotions

have arisen, and because of its perversion our daily newspapers

chronicle the basest and most sordid tragedies.

But whether noble or ignoble, this instinct for mating is, in its

simplest terms, only a provision of nature that all life contains within

itself the means of renewing life. Without this, life, so far as we know

it, would end with the present generation. Perhaps our understanding of

this decree of an all-wise nature to increase and multiply will be

heightened by looking at it not only from its familiar manifestations in

man, but more broadly. Seen from this broader viewpoint, it is the

inherent legacy of all living things from the dawn of life on the earth

down to the present. Even the simplest one-celled organisms have the

faculty of increasing. In all plants, both the flowerless ones and those

producing flowers, the process is carried to a perfection almost

unbelievable in its intricacy and in provisions against its failure.

From the matings of flowers much may be gleaned; even man himself can

learn from them the capacity for sacrifice, the sinking of individual

aims and pleasures in the greater scheme of conforming to that necessity

for renewal of the race upon which all progress must be based.

The equipment which different flowers have developed for this purpose,

their almost uncanny devices to make certain that only the distant and

foreign male can ever impregnate the female, the enormous wastage of

both unfertilized females and males that will never become fathers, and

the overwhelming effectiveness of it all, in spite of this

prodigality--these manifestations of the production of young in the

plant world will take up the rest of this chapter. All the first part

will tell of this process in flowering plants, while the second shows

how flowerless plants accomplish the same end in more secret ways.

Finally, in a brief third part, we shall see how, without mating of the

sexes, nature has still one other way to see to it that there is a

constant supply of young.

We have already made clear that all plants are divided upon the basis of

whether they bear flowers and their mating goes on before the world, or

whether they bear none and the process is accomplished in more secret

ways. Because flowers are so much better known, and it is simpler to see

how the act is consummated in them than in the _cryptogamous plants_, we

shall first consider the _phanerogams_ or flowering plants, and in the

second section of this chapter the _cryptogams_ or flowerless plants.

1. VISIBLE MARRIAGE OF FLOWERING PLANTS

In the first chapter, under the section devoted to flowers, we found

that the stamens are the male and the pistils the female organs of

reproduction. As the period for mating draws near there is developed in

the _anther_, which is the enlarged tip of the stamen, a fine, usually

yellow, powder known as _pollen_. This matures in the anther, and when

ripe is discharged from tiny pores.

Pollen is made up of individual _pollen grains_, which are very often

stuck together so that we see only the mass, not the individual pollen

grain. Sometimes the pollen is not sticky, as in the case of pine trees

or in the ragweed--a fertile cause of hay fever. In these, and hundreds

of other plants, the wind will blow great clouds of pollen through the

air. When we stop to consider that a single, or at most a very few

pollen grains are all that are necessary--in fact, are all that _can_ be

of real service--the enormous wastage of the male fertilizing substance,

in order that mating be secured, gives us some idea of how prodigal is

nature in this supreme function.

The pistil, or female organ of reproduction, is more cautious in the

expenditure of its resources. As we have seen, it is composed of a

swollen base, the _ovary_, a slender shank, the _style_, and a swollen

or branched tip, the _stigma_. In some plants the ovary is divided into

several compartments or cells, each with one or more _ovules_, which are

only immature or unfertilized seeds, often very tiny, but usually quite

easily seen if the ovary is cut open. It is the entrance of the pollen

grain into this ovule that consummates the act of fertilization. As the

ovule is carefully secreted within the ovary of the flower, and as the

male fertilizing stuff or pollen is found only on the anther, it is

obvious that some method of bringing the two together must be provided

for.

In some plants this is accomplished by the anthers being just above the

stigma, and when the pollen is ripe and the ovule ready, the stigma is

found to be covered with a sticky substance. As the falling pollen

grains touch the stigma, they are caught in this sticky substance just

as surely as flies are caught once they touch a fly paper. But just here

one of the most wonderful processes of nature begins. The pollen grain

begins, slowly at first, to grow, and in the act it penetrates the outer

coat of the stigma with a minute _pollen tube_. This slender threadlike

tube, carrying with it the male germ, grows straight down through the

stigma, into the narrowed style, and through this to the ovule. Once the

pollen is caught on the stigma, nothing is so sure of fulfillment as

that this male fertilizing stuff will ultimately reach the ovule. For

the hitherto virgin ovule this impregnation starts a new phase in life.

It means the beginning of the end, but in the process fruit and seed

will be developed, and the young bride, already a mother, has

triumphantly accomplished that for which she exists.

If fertilization of all flowers were as simple as this, there would be

no need of what follows, but actually in surprisingly few plants are the

stamens and pistils so arranged, the ripening of the pollen and

readiness of the ovule for impregnation so timed that the act can be

accomplished in such direct fashion. For it is quite obvious that in

flowers in which the whole drama of mating goes on within the petals,

without the interference or help of any outside agency, the result will

be a crop of young who know no other characters than those of the

parents, and have nothing to look forward to but a closely inbreeding

progeny, very little, if at all different from themselves. In other

words, such plants are pure bred, they lack the usually obvious virility

that comes from crossing the male of one plant with the female of

another. There are so many devices to prevent self-fertilization in

flowers, so marvelous are the contrivances to see to it that only

cross-fertilization can be effective, and, finally, the experience of

breeders that strength and virility often or usually result from

impregnating the ovules of one plant with the pollen of another, that we

are forced to the conclusion that absolute purity in the sexual

relations of flowers is rare indeed. It occurs, without peradventure of

a doubt, only in those flowers whose petals never open and where

fertilization is consummated, if not in private, at any rate without

external help. In many violets the showy violet blossoms are often

nearly infertile, while down near the ground are inconspicuous flowers

which never open, but within which fertilization is so successful that

the crop of seeds is far more plentiful than in the more showy ones that

most people think are the only flowers ever borne by violets. These

flowers that never open, or at any rate open so slightly that their

sexual processes are modestly completed without intrusion, are known as

_cleistogamous_ flowers (Figure 69). They have been found in a few

plants, but overwhelmingly the greater number of flowers not only do,

but must, rely on some outside agency to insure fertilization.

Certain structural features of flowers have been so developed that

fertilization of the ovary by the pollen of the same flower is

impossible. The commonest case is in those flowers where the stamens are

shorter than the pistils, as they always are in the common snowdrop,

hyacinth, the sassafras tree, and in hundreds of others. There can be

no consummation of the reproductive process in such flowers without some

outside aid. More futile still without this aid are those flowers where

the stamens are well above the pistils, but the time of maturing in both

differs by a few days or even hours. Nothing could be more

helpless than the pistil under these circumstances, for if

its instinct for maternity were ever so strong, it would be doomed to

barren sterility by the premature development of the males. Sometimes,

too, the female is prematurely ripe for impregnation, and the stamens

lag behind a day or two. Her time passes and with it her only chance of

fertilization--by her own haste she has rendered impotent the now

useless pollen which appears doomed to fall aimlessly upon the

unreceptive stigma.

[Illustration: FIG. 69--THE VIOLET

Note the showy, often partly infertile upper flowers and the much

smaller cleistogamous ones at the base, which never open and yet produce

a good crop of seeds.]

But, perhaps, the most hopeless of all is the well-known partridge

berry, whose red berries are common in the woods during August and

September. This seems as though it fought off any chance of securing a

mate by a flower structure and behavior that would certainly so result

if some way out of the difficulty were not at hand. The partridge berry

bears two kinds of flowers that outwardly look much alike, but whose

sexual organs differ in this way: in some flowers the stamens are all

shorter than the pistil, and in others the pistil is much outtopped by

the stamens. The extraordinary feature of it is not so much this

structural difference, however, but the fact that pollen from the

short-stamened flower is useful only to its neighboring short-styled

relative, while the pollen from this long-stamened but short-styled

neighbor is nearly useless where it is found and really useful only on

the long-styled plant. By this device, but again only with outside aid,

this plant does not prevent maternity, but increases its chances of

being fruitful, for, as we have already seen, cross-fertilization

appears to be the rule rather than the exception, and the partridge

berry not only needs it, but can exist only when its offspring are the

result of such crosses.

In all those plants that bear the different sexes in different flowers

on the same plant, as in the hickory, or even on different plants, as in

the willow, there must, of course, be some method arranged for

cross-fertilization or they would promptly die out. So general is this

cross-fertilization, so much a part of the economy of nature does it

appear to be, that we can only think that there must be in the

production of this vast horde of the cross-fertilized some advantage.

Besides securing greater virility, which almost certainly results from

this promiscuity, greater variability is promoted. If virility is the

result, the price paid for it is tremendous, for the hindrances to

self-fertilization are so many and so effective that most flowers would

inevitably die as perfectly pure but ineffectual virgins if that

fatality were not prevented. How they are saved from such a sterile

fate, how they finally secure a mate by devices that outshine the most

bewitching tricks of the daughters of Eve, is one of the most

fascinating stories in all the history of the plant world.

For, of course, flowers do secure a mate, and they are aided in this

enterprise by the most formidable array of helpers, one might almost

call them conspirators. The chief of these are insects, thousands of

different kinds of which are constant flower visitors. Some of the

smaller birds, and even snails, also help flowers to meet their mates.

The wind, too, bears pollen through the air to some expectant

bride-to-be. And, finally, in the water, by a series of acts the like of

which no one could improve for cunning, the cross-fertilization of

certain aquatic plants is consummated. It would take a book larger than

the present one to give even the briefest account of how these different

aids to maternity do their work and how the flower responds to this

help. As that is quite out of the question, only some of the best-known

examples of cross-fertilization will be given, and these will be grouped

according to what agency the flower is indebted for its chance of doing

that for which it is created.

INSECTS AS FLOWER VISITORS

On any summer day, especially when the sun is shining brightly, we may

see bees and butterflies flitting from flower to flower, busy as the

proverbial bee. We already know enough about nature’s ways of doing

things to be certain that these, and hundreds of other kinds of insects,

do not come for nothing, and that the flower must have something to

offer. Bees, especially, are thrifty creatures whose business demands

exacting and prolonged toil. They would not waste five seconds upon idle

flower calling if the blossoms did not yield a rich store. And thousands

of flowers do yield the sweetest and richest kinds of stores of nectar

or honey, which is, in fact, by the help of insects who alone can

extract it, our sole source of honey. Many flowers which produce no

nectar do have such plentiful stocks of pollen that the bees come for

that alone. In the peony, for instance, over three million pollen grains

are produced in each flower, only a minute fraction of which can ever

fertilize an ovule. All the rest would be wasted were not pollen in

itself a particularly nutritious food for young bees, and consequently

much sought after by the careful bee mothers. They are the only insects

that feed their young on pollen, or beebread, as it is called by the

beekeepers, so that the enormous overproduction of this male fertilizing

agent, from the point of view of the flower, is a decided attraction,

one might almost call it a trap, to insure constant visitations from

bees. For they are perhaps the most useful of all insects in the great

game of securing cross-fertilization, as we shall presently see. Many

other kinds of adult insects eat pollen directly and so add to the

number of insect visitors.

No one has ever been able to explain the beautiful coloring of flowers,

except that it serves as an attraction for insects and small birds. Like

the honey or nectar, it seems to play no real part in the home economy

of the flower, to be of not the least use otherwise. While honey and the

gorgeous colors of flowers are a delight to man, that would be no

sufficient reason for the ability to produce them. Both of these

attributes of flowers, as attractions for insect visitors, are, however,

so absolutely essential to cross-fertilization that we must think of

them as having grown up out of that demand. As we shall see a little

later, even the structure of some insects has been modified so that they

can reach the nectar or pollen only by automatically doing for the

flower what it cannot do itself.

While color of flowers seems as though it were attraction enough, it is

very likely that their fragrance or perfume is still more seductive in

its power of luring insect visitors and repelling useless ones. Poets

have called this perfume the soul of the flower, and in its almost

intangible beauty it might well be so called were it not for the fact

that it appears to be of not the slightest use, except as a lure. In all

the equipment of seduction there is none like this fragrance of flowers

for attracting insects.

Flowers, then, have things to offer to insects which the latter need.

Nectar and pollen are the chief, and where these merely bread and butter

objects are not enough, or sometimes in addition to them, the flower is

dressed out in gorgeous colors or perfumed with a fragrance beyond the

dreams of the fairest bride. What insects do to complete the

fertilization of such a legion of beauties makes up the romance of the

flowers. Perhaps not even in man himself is this creation of new life so

surrounded with beautiful ideas. Also, as in man, it sometimes is bound

up with an almost fiendish cruelty and cunning. Some of these visitors

and what they do, but unfortunately only a very few, can be mentioned

here. They must serve as types or examples of what is going on all about

us on any summer day.

The common blue columbine, much grown in gardens for its beautiful

blossoms, always has the flowers hanging upside down, a habit that

admirably serves to keep its pollen from rain. The opening and closing

of many flowers in cloudy weather, or at night, may be for the same

reason. Everyone knows the five blue spurs into which the petals of

columbine are produced. At the very end of each spur, which is always

curved, the flower secretes a considerable quantity of honey. This, one

of the greatest attractions to bees, leads inevitably to a visit from

one. The bee, in order to reach the honey, hangs on to the inverted

flowers, clutching the base of the spur with its foreleg, and further

securing itself by the mid or hind legs, which grasp the slender column

into which, in the columbine, the stamens and pistils are crowded. In

its anxiety to reach the honey the bee pokes its head as far into the

spur as possible, but it gets in only a fraction of the full length of

the tube. To reach the honey it extends its sucking apparatus, which is

a complicated mechanism for this purpose on the head of nearly all

insects, and which will hereafter be called by its true name of

_proboscis_. It happens that bees can easily bend the proboscis downward

or toward their own body, but only with considerable difficulty can they

bend it in the opposite way. And yet the honey in the curved tip of the

columbine can _only_ be reached by curving the proboscis to fit the

tube, and in this process the bee’s body for nearly half its length is

forced to touch the anthers. While these are close to the stigma, they

produce pollen only on their outside surface, where it is, of course,

scarcely likely to reach the stigma, but must be brushed off by the

contortions of the bee’s body in reaching the honey. The hairy body of

the bee, coated with pollen, goes next to perhaps an older flower of the

columbine. Heedless of any change in the flower the bee goes straight

for the honey in one of the spurs, again catches hold of the only

available support in the center of the flower. But this time, instead of

brushing pollen off the exposed anthers, it brushes it off its own body

to the stigmas, which, at a slightly later stage than in the one just

described, are branched and perfectly adapted for collecting the pollen

with which the bepowdered bee can hardly avoid dusting them.

Cross-fertilization is of course assured, but it seems a precarious

business at best, as the number of bees with a proboscis long enough to

do the work is limited. The columbine, by a kind of uncanny foresight,

is so constructed that bees or other insects that try to reach the honey

and are not provided with a sufficiently long proboscis, nevertheless in

further attempts upon other flowers, inevitably cross-fertilize them

without reaping their reward. One or two kinds of bees, as though in

retaliation for this subterfuge of the columbine, make short work of the

honey by biting a hole in the spur and forthwith sucking out the honey

without so much as touching anther, pollen, or stigma. The reply of the

columbine to this ravaging of its chief attraction is that finally, as a

last resort, and by a new movement of its reproductive organs, it is

self-fertilized. Here the shape of the flower, the original position of

the pistil and anthers, the exposure of pollen only in such a direction

that, while a chance of cross-fertilization still exists, it can hardly

ever fertilize its own stigmas, all point to cross-fertilization as the

plant’s greatest requirement. And yet failing this, it falls back on

self-fertilization rather than endure barren sterility.

[Illustration: FIG. 70.--COMMON BARBERRY

The stamens of this, two at a time, drive off bees by sharp blows, thus

preventing self-fertilization.]

While the columbine by its spurs and other interior structure succeeds

nearly always in holding a bee long enough to insure its being dusted

with pollen, the common barberry bush of Europe (Figure 70), also much

planted in American gardens for ornament, actually drives bees away by

sharp blows of its stamens, so that self-fertilization shall not result

from the visit. In this shrub the petals partly cover the stamens unless

the latter are disturbed, and, in fact, the curved tip of the petals

forms a kind of socket into which each of the six stamens are fitted.

The position of these is such that any insect

[Illustration: PLANTS OF THE PALM FAMILY, _Palmaceæ_, in Ceylon. They

are the talipot palm (_Corypha umbraculifera_), the fiber from the wood

of which is used in India for paper making. (_After Reinhardt. Courtesy

of Brooklyn Botanic Garden._)]

[Illustration: A GIANT HERB OF THE BANANA FAMILY, _Musaceæ_. It is the

travelers’ tree (_Ravenala Madagascariensis_), the sheathing leafstalks

of which hold considerable quantities of water--hence the name. Grown

throughout the tropical world. (_Courtesy of Brooklyn Botanic

Garden._)]

can go straight to the honey glands which are at the base of the flower,

without touching the anthers. But their filaments are broadened out at

the base, so much so that their edges touch. The honey glands are so

placed that the insect must touch the broad bases of at least two

filaments, between which, in fact, it must force its proboscis in order

to reach the honey. The moment any particular pair of filaments are

irritated by the bee, two pollen-dusted stamens fly out from their

position among the petals and the anthers strike the bee with a sharp

blow. Many observations prove that almost never does the bee go on with

his honey sucking after this rude interruption, which has resulted in at

least its head being dusted with pollen. The low, sticky stigma is so

placed that it is one of the first things the bee’s head strikes as it

reaches the center of the flower. Because of the position of the

stamens, while they are undisturbed, it is impossible that pollen from

them could have been brushed off at the bee’s entrance of the flower.

And by an almost miraculous adjustment of the power of the blow by the

irritated stamens, this drives off the intruder only after he has

brushed his pollen-laden head over the stigma. His head at this stage

is, of course, covered only with foreign pollen gathered elsewhere, but

just as soon as the bee tries to get what he came for, sometimes even

before he gets his reward, out fly the pair of stamens, thoroughly

dusting the bee, and seeing to it that the blow is just sufficient to

drive off the pollen-laden insect. No device to secure

cross-fertilization could be more effective. If the blow of the stamen

were only ever such a slight fraction less than it is, the bee would

only stop a moment and then go on honey sucking; which, because of the

release of the pollen by even the gentlest blow, would result in

self-fertilization by the aid of the bee, rather than

cross-fertilization. Very few, if any, bees will stand this gentle

reminder to go, however, and it is a little curious that such

intelligent creatures as they are supposed to be, should not realize

that it is all the clever but quite harmless trick of an apparently

still more intelligent flower to secure fertilization from any pollen

but its own.

To attract insects and then repel them seems a little like using them as

some flirts notoriously use men, only to throw them over when they are

no longer interesting. In the large-flowered magnolia tree from the

southeastern United States, insects, however, fare somewhat better than

this. In this magnolia, which has flowers several inches long,

self-fertilization is impossible as the stigmas are ready to receive a

mate several days before the laggard stamens are provided with the

wherewithal. Without some insect or other outside help there would be

only a childless old age for this particular tree. The flower opens

rather early in the season, while the nights are still cool, and as a

protection from the cold, rose beetles habitually fly into them. They

find a pleasant shelter under the three inner petals which arch over the

honey-coated stigma, and form a snug little chamber so much warmer than

the outer air that its heat is appreciable to the touch. The rose

beetles, once they are inside this warm shelter, cannot get out and are

often held for a few days. Then, as the stigma passes its period and the

stamens are furnished with pollen, the chamber opens by the gradual

withering of the petals. But the insects, in their efforts to get out,

have raised a perfect dust storm of pollen with which they are

naturally covered. Just as soon as they are released they are free to

seek another warm shelter where the process is repeated. Thus they

always enter the flower with foreign pollen, use it up impregnating the

waiting ovule, and are held until the flower’s own pollen gives the

signal for their release properly dusted for a renewal of the work. The

premature timing of the stigma, the tardiness of the anther in producing

pollen, the generation of heat and secretion of honey which frequently

covers the whole stigma--there could scarcely be a better equipment for

securing cross-fertilization. And without it the magnolia would be

simply sterile.

There are some other flowers that hold visiting insects in a trap until

cross-fertilization has been completed, and all of them by no means

furnish their visitors such a snug little heated chamber as the

magnolia. One vine from the eastern United States, known as the

Dutchman’s-pipe, or sometimes as the pipevine (Figure 71), is singularly

ruthless in this respect. Its flowers are of such evil odor that only

carrion-loving insects, such as certain kinds of flies and gnats, ever

visit them. The flower is of very peculiar structure, being formed of a

hollow tube bent from its stalk first downward, and then upward. The

upper part ends at the opening which is provided with a three-lobed lip

or doorway. Through this the insects crawl, and they finally reach the

bottom of the curved part of the flower. Behind and above them is the

entrance through which they have just come. And above them, in the other

curved part of the flower, is the stigma. As in the magnolia, this

matures several days before the pollen from its surrounding stamens is

ripe, so that self-fertilization is never possible. What is now the

[Illustration: FIG. 71.--THE DUTCHMAN’S-PIPE

A vine producing evil-smelling flowers, which trap insects sometimes for

days, thus insuring cross-fertilization.]

plight of the insect at the bottom of the upward-pointing tubes, one

leading to the organs of reproduction, the other to the exit? By an

almost diabolical cunning the inside of this flower is so smooth that no

insect can crawl up its slippery sides. It takes some time for the

prisoner to find this out, and, in the meantime, it has explored every

nook and corner of the flower by flying. In the course of this

exploration it reaches and covers the stigma with pollen, for as we

shall see presently, it always comes _into_ the flower pollen-laden.

Evidently becoming panicky about getting out, the insect then flies with

very considerable force in every direction. Toward the true exit it

naturally flies the most, and by a refinement of cruelty this is the

lighter end of the flower, and therefore the obvious mode of escape for

it. But the three lips and the entrance they cover are not flat across

the flower, as the cover of a lunch box would normally be, but turned at

such an angle as the lunch box would be if set on end. The insect, in

flying toward the light, invariably hits the smooth surface just inside

the three lips and falls to the bottom of the flower because there is

nothing rough enough for it to cling to. Throughout most of the day, and

sometimes for several days, the insect will keep up this ceaseless

struggle to escape, flying first up one tube and then the other. These

frantic efforts would end in exhaustion if kept up too long, and before

that happens, but after the impregnated ovules have no further use for

the flies, the anthers give forth plentiful supplies of pollen. Of

course, the insect can scarcely avoid becoming covered with this, and

then, but only then, the flower begins to wither and up its now wrinkled

sides the pollen-laden prisoner can at last crawl to liberty.

There appears nothing very romantic about the cross-fertilization of the

Dutchman’s pipe, in fact, the whole affair seems but a sordid and, it

must be confessed, a very efficient trick to get what the flower needs

from the insect, rewarding it by many hours of apparently hopeless

captivity. But most flowers do have something that insects want, and

none so well fulfill the expectations of butterflies as the meadow pink.

This is a graceful little perennial native in the fields in central

Europe, but often grown in American flower gardens. It has beautiful

pink flowers, with a long tubular calyx, at the bottom of which are rich

honey glands, accessible only to the long proboscis of different kinds

of butterflies. No other insects, and many try, are able to get the

honey from this plant. It begins flowering by opening its five

beautifully fringed petals, all of which are marked with lines from

their edge toward the center. These obvious “pathfinders” are common on

many other flowers and all seem to be there to act as a guide for

alighting insects, and, as it were, steer them to the center of the

flower. As a butterfly alights he finds five protruding anthers covered

with pollen. These are of no use to him, but nevertheless his head is

covered by their pollen. In fact, many other visitors, who can never

reach the honey, find the bright color and good stocks of pollen

sufficient attraction for them, and after a pollen feast fly off to

other flowers. If the butterfly comes at this state of the flower’s

life, it pokes its long proboscis down into the tube of the calyx, but

finds the passage almost blocked by another set of five stamens not yet

ready to discharge pollen, and, as though ashamed of the fact, quite

hidden from view. Also, the proboscis is very nearly stopped by the

style, which, if its stigmas were ready for mating, would then and there

become impregnated. But they are rolled together into a tight spiral,

and their pollen-receptive faces tightly pressed together in addition to

the whole structure being twisted, so that the very probably

pollen-laden proboscis of the butterfly finally gets to the honey

without leaving one grain of pollen where it could possibly

self-fertilize the flower. Honey-laden, it now flies away, and so far as

this particular flower is concerned, simply nothing has happened but to

coat several insects with pollen. A little later the anthers of the

first set of stamens cease work and drop off. Then the five stamens

hitherto hidden in the calyx burst out and furnish a second crop of

pollen, but the stigmas are still safely coiled away from the possible

danger of self-impregnation. During this second stage the flower may

repeatedly be visited, but until this second crop of anthers become

useless there is not the slightest risk of the stigmas becoming

self-fertilized. For the hole through which the second set of stamens

has come is so small that still only the proboscis of the butterfly can

penetrate it. When finally the second crop of anthers also fall off,

then the style slowly uncoils and thrusts its now receptive stigmas

above the calyx and in plain view of passing visitors. These come,

pollen-laden of course, from other flowers, and cross-fertilization is

assured. Here the flower furnishes two crops of pollen, plentiful

supplies of honey, and asks only that in gathering these the butterflies

see to it that its stigma be covered only with foreign pollen. Keeping

carefully out of the way while they are about their business and there

is danger of self-fertilization, they come out boldly once that danger

is past. In this meadow pink self-fertilization is simply impossible;

everything in the production of its young it owes to the butterflies, to

which it surely makes adequate returns.

While such plants as the meadow pink and thousands of others have lost,

if they ever possessed, the power of self-fertilization, and rely

absolutely on insect visitors for their perpetuation, there are many

hundreds of kinds that apparently hope for cross-fertilization, but, in

default of it, due to their inability to absolutely compel it, they

finally accept self-fertilization as a last resort. Darwin once said

that “Nature abhors perpetual self-fertilization,” and the frequent

visits of insects and their rôle in preventing it, together with the

flowers’ adaptations to such visits, support the contention, which has

never been seriously questioned. But some flowers appear to have left

the back door open, as it were, so that failing cross-fertilization,

they may still rely upon self-fertilization. A geranium from the

Pyrenees, a relative of our common woods geranium, is of this type. A

day or two before its stigmas are ripe it produces first one set of five

anthers, all pollen-coated, and then another. If these have not been

brushed clean, as often or usually happens, then the stigmas ripen and,

of course, are impregnated by their pollen. If they are brushed clean of

pollen, then the stigmas must rely on foreign pollen, of which it is

assured a supply from the visits of pollen-laden insects from other

flowers, which are still attracted by the flowers’ color and honey.

Perpetuation is assured, in any case, but the preference is still for

cross-fertilization.

A more remarkable case of leaving one final chance for

self-fertilization is the gas plant. It exhales such a strong and

peculiar odor that only certain kinds of insects will visit it. In fact,

the odor is so strong and is so heavily charged that a lighted match

held near it has been known to slightly ignite--hence the plant’s name.

The flower bears a low, squat stigma, profusely covered with honey,

which is perfectly accessible to any insect visitor. It has ten stamens,

which at first are quite out of the way of insects, two being folded

back in each of the five yellow petals. First one stamen begins moving

gently from the shelter of its petal, and the anther, pollen-coated,

hovers over the stigma, which would inevitably lead to

self-fertilization if the stigma were only ready. It never is, and, as

though realizing this, the stamen gently moves back out of the way,

still, in most cases, retaining some of its pollen. Then another tries

and, again, as though realizing the futility of impregnating the

unreceptive stigma, it also moves back. So it goes with all the other

eight stamens, each of which moves gently out over the stigma, and

gently back again, all of them failing to fertilize the laggard stigma.

But there has been during all this time a constant procession of insects

coming for the honey, and never for a moment have the stamens been off

guard, so that each visitor goes off with at least some pollen clinging

to it. Finally the stigma, after all the faithful ten are folded back

among the petals, comes into its period, and is cross-fertilized by the

insects which come from other flowers laden with pollen, as we now know.

But if, by one of those accidents of nature, such as bad weather or what

not, no insect ever does come, what then is to be the fate of the

stigma? Has it staked all only to lose out in the end? It would almost

seem that to ignore the steady attentions of the willing ten might make

barren sterility a fitting punishment. But, in spite of what has

happened, the stamens come to the rescue, if all insects fail, and this

time, in a body, they rise up and shed upon the stigma from the

half-withered anthers their few remaining pollen grains. There could be

no finer example of having an anchor to windward.

One of the largest families of flowering plants is the pea family

(Figure 72), with over five thousand members, practically all of which

rely on bees for cross-fertilization. In some kinds, where bees

occasionally fail them, the flowers wither without self-fertilization

and, of course, no seed are then produced. In such a large family of

plants there are

[Illustration: FIG. 72.--THE EVERLASTING OR PERENNIAL PEA

A member of the Papilionaceæ or pea family which rely almost entirely on

bees for fertilization.]

naturally many different adaptations for securing

cross-fertilization--some of them of such extreme complexity that they

could hardly be included here. All the family have the characteristic

pealike flowers familiar enough in the sweet pea, which have already

been described and figured on page 44. In all of these stamens and

pistils are hidden inside the keel, at least in the early stages of the

flower. In some, such as clovers, for instance, the organs emerge from

the keel, and after fertilization by insects re-enter their retreat.

There are scores of different plans for securing the desired object, but

the common alfalfa, with a few other related plants, has the most

startling. The flower begins life with its stamens and pistils

concealed within the keel, which is apparently impregnable. When a bee

alights on the flower and begins work he is welcomed by a small but

violent explosion. When the dust of this clears away, and it is actually

dusty with pollen, the dazed bee is seen to have fertilized with almost

instantaneous rapidity the stigma of the exploding flower, which springs

violently out of the burst keel. This is so arranged that, as it flies

out of its trap, due to the explosion touched off by the bee, it strikes

the under side of the bee’s body a distinct blow, brushing off in the

process the pollen nearly always found there. This pollen has come from

other plants. But, as in most plants of the pea family, the stamens

closely surround the stigma, but are shorter than it. When the explosion

occurs, the stigma, because it is slightly longer than the stamens, is

the first thing to strike the bee’s body. Already impregnated, it is

then indifferent to the cloud of pollen from the stamens of its own

flower, which only a fraction of a second later also strikes the bee’s

body. The first flower that a bee visits cannot, of course, be

cross-fertilized notwithstanding the explosion which results, no matter

from what angle the bee attempts to insert its proboscis. But in this

case, as the stigma is unfertilized by foreign pollen, its own performs

that service. In the vast majority of cases cross-fertilization is

assured by certainly the most novel of processes, and, in the rare event

of the bee not being covered with foreign pollen, self-fertilization is

still possible.

In many of the plants already noted cross-fertilization is accomplished

by virtue of the fact that the stamens mature before the stigma. But in

the common strawberry the reverse is true. As the insect stands on the

white petals, it must, in order to reach the honey at the base of the

flower, put its head down so that if pollen were available there could

only result self-fertilization. But while the stigma is ready, the

pollen at this stage never is, and the insect which comes usually laden

with pollen from other flowers cannot avoid impregnating the stigma with

this foreign pollen. Later the stamens mature and, as insect visits

continue so long as honey is to be found, they become dusted with pollen

which is used for the fertilization of other plants. Scores of other

plants also produce ripened stigmas before the pollen matures, and they

must rely on insects to cross-fertilize them.

The cross-fertilization of the strawberry is such a comparatively simple

process--seems in fact almost inevitable--that we are lost in wonder at

the almost mathematical complexity of the act in the common purple

loosestrife, which has been introduced into American gardens from

Europe, and sometimes runs wild. In this plant there is a long terminal

spike of showy, purplish-pink flowers, the color of which is sufficient

to attract many insects from even a fairly swift flight. The petals are

streaked with “pathfinders” toward the center of the flower. This

consists of a tubular calyx; at the bottom of this is the honey, which

secures the insect’s further interest once the color has attracted it.

But it finds a condition of the reproductive organs almost without

parallel. In some plants the style is hidden down in the calyx tube,

while one set of stamens just peep out of the end of the tube, and a

second set are still further and quite obviously protruded. In a

neighboring plant the style will be found outside the tube, one set of

stamens hidden in it, and the other set outtopping everything, except

the petals. In still a third type of the loosestrife, the style exceeds

everything but the petals, one set of stamens just emerge from the tube

and the lowest set are hidden. It would appear as if the loosestrife

could scarcely escape self-fertilization, except possibly in that form

where the stigma outtops all the stamens, and this would result always

if the pollen were indiscriminately useful from all three lengths of

stamens. But it never is, only that from the short-stamened plants will

fertilize the short-styled one, the mid-stamened ones the mid-styled

counterpart, and the long-stamened ones the long-styled flower. The

pollen grains of the different-lengthed stamens are even of different

size and color. If this were not so, it is a simple mathematical problem

that, with three different sets of style lengths and six sets of

stamens, two in each flower, eighteen different crosses might be

possible. As it is, only six crosses are ever possible and these only by

the aid of insects, for it must be remembered that stamens and pistils

of the same length are never found in the same flower. By an adjustment

of the size of the body of the different insect visitors it works out so

that, while all three body sizes frequently visit each flower, only that

particular size of insect suited to the carrying of pollen from stamens

of one definite length to a style of similar length actually

accomplishes the cross-fertilization of that particular flower. And this

without interfering with the visit of another different-sized insect

which will accomplish the work for its particular set of stamens and

pistils. So marvelous is the adjustment of style length and stamen

length to each particular body size of the visiting insects, so perfect

is the arrangement of the organs in each flower, that each contributes

to the fertilization of its neighbors, never to its own, nor does it

interfere with the process in other lengthened styles.

Cross-fertilization is thus almost always accomplished in the six

different combinations possible in this truly remarkable case of

adaptation between insects and flowers.

From the almost mathematical complications of the cross-fertilization of

the loosestrife it seems a far cry indeed to that of the Italian

honeysuckle. This often runs wild over fences, but is unlike the more

widely known Japanese honeysuckle, in that its stem passes through the

different pairs of opposite, bluish-green leaves, which are joined

together at the base. The Italian honeysuckle falls back on the more

simple seductions of odor and honey for securing its really important

insect visitors. It has such a long tube that only certain night-or

evening-flying moths or butterflies can reach the honey. There is, even

during the day, such a plentiful supply of this that it frequently fills

half the tube, but even then it is quite out of reach of bees which

never succeed in getting any. Toward evening, particularly on quiet,

still evenings, the flower begins to send off in much increased quantity

a heavy rich-scented odor almost overpowering in its sweetness. The

butterflies and moths of the dusk having a long proboscis, succumb to

this really enchanting lure, which, with the large store of honey,

insures quantities of eager suitors. The stigma, while ripening

simultaneously with the anthers, protrudes beyond them, so that the

butterflies touch pollen only _after_ touching the stigma, which of

course is impregnated with pollen from an earlier visit of the insect to

a different flower. So assiduous are the butterflies, that on a still

night there will be not one grain of pollen left on the much-brushed

anthers. If the night is cold or windy much pollen remains and

cross-fertilization is left to pollen-eating insects such as mother bees

or flies. While these can never get the honey they often do accomplish

cross-fertilization and, sometimes by misadventure, self-pollination. It

is only those moths and butterflies which, forcing their long proboscis

down the honey-laden tube, must accomplish cross-fertilization. But

failing this, the plant is more or less at the mercy of hosts of insect

triflers, mere pollen eaters, who may or may not insure

cross-fertilization, but in any case provide for self-fertilization.

Another use which certain plants make of honey, besides acting as a lure

of insects, is found in the common lilac. The flower in this has

considerable honey at the bottom of the tube, which can only be reached

by insects with a proboscis sufficiently long to reach it. The lilac has

only two stamens inserted near the top of the tube, the passage to which

they very nearly obstruct. The stigma is hidden in the tube, and it

matures simultaneously with the ripening of the pollen. As the insect

inserts its proboscis between the stamens no pollen clings to it due to

the character of the pollen grains. But as the proboscis is withdrawn

from the tube its lower end is covered with honey to which pollen

sticks. If a needle is inserted between the stamens and pushed only far

enough to be still clear of the honey, no pollen will be found on it

when withdrawn, but if pushed all the way down, its honey-coated point

will catch considerable pollen. In the lilac, if insect visitors do not

accomplish the work of cross-fertilization, the flower is

self-fertilized ultimately by the protruding of the stigma far enough

out of the tube to catch some of the remaining pollen grains.

It is perhaps useless to multiply instances of flowers which by various

devices secure the cooperation of insects in getting pollen from a

foreign source. To recapitulate some of those devices it is necessary

only to recall what some flowers have done to force cross-fertilization.

The heated chamber of the magnolia, the cruelty of the prison cell in

Dutchman’s-pipe, the blow from the stamen of the barberry, the faithful

rotation of the ten stamens of the gas plant, the explosive flower in

some members of the pea family, the lure of honey and seductive odor of

the Italian honeysuckle, the mathematical complexity of the

loosestrife--these and hundreds of others all point to the necessity of

cross-fertilization and a means to produce it almost beyond belief.

There is the best of evidence that not only flowers but insects

themselves have been modified in this great work, and that for every

flower needing cross-fertilization some agency has been developed to

secure it. Insects, beyond all other animal life, do this work, but it

is accomplished by humming birds often, and in one plant even by a

snail.

Two of the very largest plant families, not so far mentioned in this

account, depend almost absolutely upon insects. In the daisy family,

with over eleven thousand members, the large heads of flowers, often

containing scores of individual flowers, are constantly brushed, over

and over again, by the pollen-coated bodies of insect visitors. And in

all or nearly all orchids (Figures 73-75), comprising over five thousand

kinds, the same process is accomplished. In these plants, in fact, the

act is, if possible, more

[Illustration: FIG. 73.--PINK LADY’S SLIPPER

(_Cypripedium acaule_)

A native orchid in northeastern North America.]

complicated than in any so far noted. Darwin’s book, “On the Various

Contrivances by Which British and Foreign Orchids Are Fertilized by

Insects,” reads like a fairy tale. Yet it is the result of years of

patient observation by incomparably the greatest naturalist of recent

times. To it the reader must go for the details of a drama of absorbing

interest, but too long to sketch even briefly here. Perhaps one

illustration may be mentioned of how far the principle of

cross-fertilization has been carried, and to the deadly effects of its

failure in at least one case. In a certain orchid from Brazil, known as

butterfly orchid, the pollen is nearly always carried out of the flower

by an insect visitor, but, if by mischance it is not, and falls on the

stigma, not only does it fail to fertilize the ovules, but it kills the

pistil forthwith. There may be a few other cases of such drastic

results of self-fertilization, but in any case, and disregarding these

apparently suicidal fanatics, cross-fertilization is so very nearly

universal that nature must find it of enormous advantage. Only in this

way is it possible to explain the intricate adjustments of insects and

flowers, which work together in such wonderful harmony that

cross-fertilization hardly ever fails in those flowers where it appears

to be necessary.

[Illustration: FIG. 74.--SHOWY ORCHID

(_Orchis spectabilis_)

Native of eastern North America, with showy magenta-pink or white

flowers in a loose raceme.]

It must have struck many thoughtful readers to ask a rather obvious

question at this point. Why, if untold millions of insects are

constantly flitting from flower to flower, does not the pollen get

mixed, as it is quite certain that they will not fly from a

[Illustration: FIG. 75.--RATTLESNAKE PLANTAIN

(_Peramium pubescens_)

One of the few orchids native in eastern North America, with white

variegated leaves. It grows in dense patches and bears free-blooming

spikes of whitish flowers.]

certain kind of geranium to another similar one for instance, but

perhaps to a rose? The answer to this is simple enough, but its

implications are limitless. Only pollen of a certain species or variety

is useful to the stigmas of that variety. To practically all others the

stigma is simply unreceptive, except in those closely related plants

that may all have a common parentage. When crosses between such closely

related plants do occur the result is known as a hybrid, which will be

considered elsewhere. To this extent, then, flowers are peculiarly

exclusive in their matings and promiscuity occurs in the vast number of

cases only in plants of the same species. We do not yet understand the

impotence of pollen of one species upon another; all that we do know,

which has many times been proved by experiments, is that it fails to

act. If it did act, no one could picture the chaos into which the

vegetative world would be thrown.

WIND AND WHAT IT DOES FOR FLOWERS

While, as we have seen, thousands of plants rely upon insects for

producing their young, still other thousands put everything to the

hazard of the wind. Pollen is so light that it can easily be blown very

great distances, and while the wastage is enormous, the process works so

well that the greater part of the vegetation of the earth is thus

fertilized. This is true not as to the number of different kinds of

plants, for in that respect insect fertilization is more important than

that accomplished by the wind. But in the number of _individual_ plants

concerned the wind is incomparably the greatest fertilizing agency that

is known to us. This for the reason that all grasses and sedges, most

catkin-bearing trees such as oak, hickory, birch, practically all pine

trees and their relatives rely wholly on the wind for fertilization.

For reasons that will be enlarged upon in another chapter, all of these

great groups of plants must be considered as of simple structure, some,

like the pines, relics of a remote past when no flowering plants, as we

know them to-day, existed on the earth. In any event the reliance upon

the wind is certainly hazardous, and while it of course insures nearly

universal cross-fertilization, it may well result in scanty

fertilization or, in exceptional cases, complete failure of it. Quite

obvious also is the amount and direction of the wind in the process, for

in very open and windy places grasslike vegetation, or at least a

predominance of species fertilized by wind, is likely to be found,

rather than those plants that rely upon insects, that, unable to stand

the full force of the wind, seek more sheltered places. While such a

thing is not the cause of prairies, or the predominantly grasslike

vegetation along sand dunes, or the exclusive spruce forests of the

bleak and windy north country, it unquestionably aids in maintaining the

often exclusive nature of such pure associations of plants. Over

thousands of square miles on our own great plains or on the steppes of

Russia, both subject to violent winds, the great bulk of the vegetation

is wind fertilized. It could hardly be expected that pollen, once in the

grip of such a wayward and shifting thing as the wind, should not be

wasted in great quantities. This is particularly true of pine trees,

which at pollen time may often be seen giving off golden clouds of dust,

of which perhaps 95 per cent is wasted.

WATER AS AN AID TO FERTILIZATION

Those submerged aquatic plants upon which neither the winds nor

honey-seeking insects can work the magic of cross-fertilization, seem to

be about the poorest equipped for perpetuating their kind through

impregnation of their tiny flowers. And yet, for at least two of them,

which will be described presently, the process is accomplished by an

adaptation of their mode of life to their watery environment that seems

incredible. These two have been selected as illustrating two peculiar

adaptations in the weight of pollen or pollen-holding flowers that is

common to some other submerged aquatic plants. In one the male flower,

or pollen from it, with the very nicest adjustment of function to

environment in all the realm of the plant world, is just of the right

specific gravity to float to the surface with dramatic suddenness and

perfectly timed effectiveness. In the other the pollen is _just_ enough

heavier than the water to float betwixt the surface and the bottom, so

that at the proper moment it is where it can fulfill its destiny.

The common eelgrass or tapegrass is a submerged aquatic which roots in

the mud and has long grasslike leaves which may often be seen waving

gently in the current of many quiet streams in this country and in

Europe. Down near the base and in among its swaying verdure, it bears

tiny flowers which have no petals, and in which, as if recognizing the

futility of display in such a secluded watery home, even its calyx is

reduced to small scales. Some of these minute flowers are females,

others again all males, and as they appear in their early stages it

looks as though never the twain could meet. And the hopelessness of

their ever meeting is increased as the maturing female begins slowly to

uncoil the fine stalk upon which it grows. Steadily but surely the loose

spirals of the stalk of this ever more mature female flower uncoils,

until, when quite ready for the pollen, it is at last upon the surface.

The male flowers, in the meanwhile, are down near the bottom with their

small freight of pollen ready to perform their function, but firmly

anchored to a stalk absurdly inadequate to reach the surface where alone

they can be of service. A great Belgian, Maurice Maeterlinck, who

studied this plant with more sympathetic vision than any botanist has

yet been able to equal, wrote in one of his essays on “The Intelligence

of Flowers” the solution of this little drama of apparent hopelessness.

No other words can ever convey the meaning of what happens to the

eelgrass quite so well as his. “Is there any more cruel inadvertence or

ordeal in nature? Picture the tragedy of that longing, the inaccessible

so nearly attained, the transparent fatality, the impossible with not a

visible obstacle! It would be insoluble, like our own tragedy upon this

earth, were it not that an unexpected element is mingled with it. Did

the males foresee the disillusion to which they would be subjected? One

thing is certain: that they have locked up in their hearts a bubble of

air, even as we lock up in our souls a thought of desperate deliverance.

It is as though they hesitated for a moment; then, with a magnificent

effort, the finest, the most supernatural, that I know of in all the

pageantry of the insects and the flowers, in order to rise to happiness,

they deliberately break the bond that attaches them to life. They tear

themselves from their peduncle, and, with an incomparable flight * * *

dart up and break the surface of the water. Wounded to death, but

radiant and free, they float for a moment beside their heedless brides

and the union is accomplished, whereupon the victims drift away to

perish, while the wife, already a mother, closes her calyx, in which

lives their last breath, rolls up her spiral, and descends to the

depths, there to ripen the fruit of the heroic kiss.”

In the eelgrass it is the specific gravity of the male flower, or, the

secreted air bubble, which makes the flower lighter than the water, and

actually causes the flight from the depths to the surface. Because of

this, fertilization can only take place on the surface, although the

flowers and fruits otherwise mature under water. But in sea wrack, in

Naias, and in ditch grass, all submerged aquatics, the flowers are even

fertilized under the water. Pollen in such plants is much modified, and

instead of being in the ordinary form of pollen grains, it is, at the

time of ripening, lengthened out into tubular, hairlike structures.

These delicate prolongations of the male fertilizing stuff are carried

by the currents of the water, just as a thread would be, but with the

difference that the pollen threads are so beautifully weighted to fit

their watery environment that they float, suspended, in the depths of

the water at or near the level of the female flowers. The pollen is set

free at maturity, just as it is in the eelgrass, but to meet the female,

which never rises, it must float with the current of the stream. There

must, as in the wind-carried pollen, be a tremendous wastage, yet

sufficient quantities of it do fertilize the females, particularly in

the ditch grass, which fruits very freely.

Whether it be any of the various contrivances for insect fertilization,

or by the winds, or, as in the eelgrass, by the water, the climax of the

flower’s life is always reached in this act. For all annuals the plants,

also, begin to die down then, a process that is completed with the

production of seed, which is, of course, the object of all those varied

modes of fertilization. Perhaps no answer to the question of why plants

do not always self-fertilize themselves is so eloquent as the hundreds

of ways they have adopted to avoid doing so, a few of which we already

know. Many volumes have been written on this subject, but all of them,

intricate as the methods they describe nearly always are, merely confirm

what we have already seen--that rather than submit to

self-fertilization, plants will adopt almost undreamed-of expedients.

Sometimes, as in the eelgrass and in the visits of nocturnal insects to

those night-blooming flowers that carry on their matings in the glamour

of moonlight or in the dusk of eventide, the drama, in the eyes of

imaginative writers, is one of singular beauty and charm. And, on the

other hand, we have seen the well-nigh heartless cruelty of the

Dutchman’s-pipe in keeping as prisoners its absolutely necessary insect

deliverers. Even this is outranked for matchless ruthlessness by a wild

arum, a relative of our jack-in-the-pulpit, from the East Indies. It

produces a club-shaped inflorescence composed of tiny flowers that need

cross-fertilization, but so offensive is the odor of the flower that no

insects will tolerate it. A snail, a voracious eater of foliage, is

attracted to the flower partly by the fine fleshy leaves, but mostly by

a juice secreted at the apex of the flower column. To this the snail

crawls, and fertilizes the tiny flowers over which it drags its body.

When this is accomplished it speeds on hungrily to the juice just above

it and eagerly devours the poison. Death follows almost immediately. The

secretion of this murderous liquid to lure the only creature that will

visit such an offensively malodorous plant, which, without it, would

very likely be itself destroyed by the foliage-eating snails, is a

gruesome contrast to that happy flitting of butterflies which completes

the fertilization of most flowers in equally effective but more pleasing

fashion.

Once impregnation of the ovule has been consummated, it begins a slow

process of change, involving sometimes the modification of the ovary, or

of the calyx, and very often of the swollen apex of the flower stalk

upon which these organs are borne, known technically as the

_receptacle_. We have seen, in the first chapter, what greatly different

types of fruits are developed from different ovaries, and they of

course produce seeds in varying size and amount. In the coco de mer, a

palm from the Seychelles, the seed often weighs forty or fifty pounds,

while in some orchids a single capsule will contain over a million

almost microscopic seeds. Some of the devices of fruits and of seeds to

secure the utmost spreading of the species over the earth will be

considered in another chapter. All the devious methods of plants in

producing their young become significant, so far as the earth’s

vegetation is concerned, only when we find out what this enormous

progeny has done with their opportunity. The chapter on the Distribution

of Plants will tell us how well that opportunity has been used.

2. HIDDEN MARRIAGE OF FLOWERLESS PLANTS

As we stated in the first chapter _cryptogams_, while they produce no

flowers, must bear organs that perform the _functions_ of flowers in the

reproduction of new individuals. Because, generally speaking, the

process is more hidden in its manifestations, and nearly always requires

the aid of the microscope to detect it, it is not so well known as the

reproductive processes of flowering plants by those who have not the

opportunity to manipulate such instruments. The act, however, is just as

interesting, and, as we shall presently see, it may well be considered

the ancestor of those more showy methods of producing young, which have

been all too inadequately treated in the preceding pages. While the

parts having to do with reproduction in flowerless plants are

microscopic in size, it is possible to understand the broad outlines of

what goes on and perhaps the life history of such plants is as well

illustrated in ferns as in anything else.

THE LIFE HISTORY OF A FERN

In the discussion of ferns in the first chapter we found that on the

back of some of their leaves, or occasionally on special leaves devoted

to the purpose, were many small brownish or dark spots, arranged in

rather definite fashion, and known as _sori_. (Figure 63.) Each sorus

contains many minute bodies known as _spores_, not unlike very miniature

seeds in general appearance, but quite unlike them in behavior and mode

of life. No better idea of their size can be gleaned than to record the

fact that in each sorus there may be about one hundred small, often

short-stalked spore cases, known as _sporangia_, and that in each

sporangium well over forty, and sometimes over sixty, spores will be

crowded. A healthy specimen of many of our common ferns will bear about

ten or a dozen leaves, each of which is divided into many divisions, and

among these divisions of the leaf there may be at least fifty that bear

from fifteen to twenty sori. It can be easily figured from this that a

healthy plant of this fern may and usually does produce over forty-five

million spores, each of which contains within it the opportunity of

developing into a new plant. There is thus a prodigality in producing

the means of renewal of life among ferns that far outstrips the

production of seeds in even the most prolific of flowering plants.

When the spores in the sporangium are mature and therefore ready for the

next stage in their life history several things must happen. With

somewhere about six thousand of them crowded together under each sorus,

more room to develop is obviously the first consideration. This is

provided for by the fact that when the spores are ripe the sporangia

have the ability to throw them considerable distances; then of course

the wind can carry them much farther. To be of any use they must fall

upon damp ground, for some degree of moisture is absolutely necessary

for what is about to happen to them. In nature countless millions never

do fall in a favorable location, or, if they did, such an enormous

production of fern spores would soon make the world exclusively a fern

garden. The comparatively minute fraction of them that ever do find

congenial surroundings, once they are expelled from the spore case, then

begin a process that is not unlike the germination of a seed. For the

spore must take in water from the soil, which by osmotic pressure

finally bursts it open. From the burst spore a minute tube, known as the

_protonema_, or literally first thread, begins to develop. It is, of

course, of microscopic size, and yet near its base there is a branch

tube formed, differing from it in structure and ultimately forming

_rhizoids_, which are rootlike hairs. Both the protonema and the

rhizoids begin growing, the first forming, usually flat on the ground,

an often heart-shaped body having the characteristic green coloring

matter of all plants. The rhizoids multiply and look not unlike roots.

This young, still microscopic plant, grows apace, and may soon be

distinguished with the naked eye. It looks not unlike a heart-shaped

mass of greenish tissue quite flat on the ground, and is called a

_thallus_.

Up to this point, then, we may trace the story of any fern which has

thrown off its cloud of spores and from which develops this tiny

thallus, looking not in the least like a fern nor as though it could

ever be modified into one. Because this thallus is,

[Illustration: FIG. 76.--PROTHALLUS OF A FERN SEEN FROM THE UNDER SIDE

(_A_^{1}) archegonia, (_A_^{2}) antheridia, and (_A_^{3}) the rhizoids.

_B_: Prothallus, showing the young plant with its first leaf (_B_^{1}),

its own roots (_B_^{3}) and the rhizoids of the prothallus (_B_^{2}).

Drawing and legend for it slightly altered from Kraemer.]

in the truest sense, merely a preparation for the process that _will_

produce another fern, it is always known as a _prothallus_. The

prothallus is thus the first stage in the reproduction of ferns, a very

simple stage, with only the faintest indication that the thallus might

be considered the vegetative and its rhizoids perhaps the rootlike

counterparts of foliage and roots of mature ferns. As we shall see

presently, even this differentiation has not the significance that such

a structure in flowering plants would indicate. There is not, as yet,

the faintest indication of sexes that need to mate in order to produce

their young. The spore has so far only produced a tiny flat body of

green tissues with a few rootlike threads, so unlike the fern from which

it started that its true significance, or even the fact that it had

ought to do with ferns was not known until about the middle of the last

century.

This green cushiony prothallus keeps on growing, its heart-shaped mass

becoming divided into an obviously left and right hand side and the

rhizoids multiplying in number. They are always borne on the lower side

next the ground, or next whatever the prothallus may be growing on. Near

the notch of the heart-shaped prothallus are developed a few

flask-shaped bodies which contain within them an egg cell or single

ovum, the female reproductive body. By a series of changes this egg cell

becomes embedded in a mucilaginous material. This flask-shaped body with

the female egg cell inside is known as the _archegonium_. From among the

rhizoids there may, at about the same time, be found developing small

globular organs that have in them a number of tiny cells, each of which

has attached many minute threadlike tails. The globular organs, with

their minute, tailed cells are known as _antheridia_, and comprise the

male reproductive equipment. Just as in flowering plants, neither the

_archegonia_ (female) nor the _antheridia_ (male) can produce offspring

without mating and the method by which this marriage is accomplished

differs tremendously both in practice and in its significations from

that in phanerogams. In the first place, the male and female

reproductive cells are separated by a considerable distance, they are

both inclosed in structurally different casings, and the whole operation

is so microscopic that insects can be of no service. Nor can the wind do

for them what we have seen that it does for the pollen of pines and

grasses.

Of the aids to fertilization there remains then only the water, which

plays such an important part in the mating of the eelgrass and ditch

grass among flowering plants. But in these ferns a very different drama

is about to be enacted. The male cells, as we have seen, are provided

with slender tails, which are movable. They move, in fact, to such good

purpose that the male cell can actually swim in the water. Of course its

minute size demands only the merest drop of water, in which it will take

the only excursion of its brief life. For just as soon as it is mature,

a heavy dew or the tiniest particle of water will set free the little

male messengers. The water too has not been without effect on the female

cell. More remarkable still, this mucilaginous matter contains in it a

substance that acts as a lure to the swimming male cells. In any event

they do swim directly to the entrance of the female cell’s abode,

through it and to her, when the union is effected. At once there is

thrown across the entrance a membrane that excludes all other males, and

the fertilization is complete. From this union of the male and female

cells a true young fern begins to develop. First a young leaf and roots,

finally a stem and in the end, of course, a full-grown fern producing

spores, ready to renew the whole process.

Some ferns do not follow all the steps exactly as we have outlined, for

all of them have not the structure of the typical one whose life history

has been sketched above. In the adder’s-tongue fern, for instance there

is a stalklike prolongation from the base of the only leaf the plant

bears, on which all the spores are borne. In certain others, as in the

ostrich fern, the spores are borne on leaflike growths that serve only

this function. Most ferns, however, bear spores on otherwise unmodified

foliage leaves and the great bulk of them on the under side of such

leaves.

There are several things about the life history of a fern that differ

fundamentally from any flowering plant and perhaps the chief is what is

known as the alternation of generations. A spore, for instance, can

never produce a fern as a seed will always produce a flowering plant. In

this respect they are like many insects that always have two or

sometimes three different stages in their life history. Only by the

complicated method of first a spore then the prothallus, from which

archegonia and antheridia are produced, followed by the free swimming

male cells fertilizing the female, can a fern reproduce itself. As we

shall see in the chapter on the History of the Plant Kingdom, this

alternation of generations, the absolute necessity of water in which to

carry on the fertilization, and above all the ability of the male cells

for free swimming in the water, are all landmarks in the development of

plant life. In its simplest form fertilization in flowerless plants is

characterized by one or all these processes, as it is in the ferns,

while in the flowering plants, the act is accomplished by processes,

discussed previously, which, in the development of the plant kingdom,

mark a period only comparable, in the history of man, to such tremendous

achievements as the acquirement of speech or the ability to make a fire.

LIFE HISTORY OF A MOSS

Ever since the war, the peat-forming mosses, known as sphagnum, have

become more widely known to the general public than any of the ten or

twelve thousand mosses known to grow on the earth. Its power of

absorption, greater than linen bandages, made it extensively used to pad

surgical dressings. Hundreds of thousands of these sphagnum

[Illustration: A SMALL CLOUD OF WIND-BLOWN POLLEN OF THE JAPANESE RED

PINE (_Pinus densiflora_). (_Photo by C. Stuart Gager. Courtesy of

Brooklyn Botanic Garden._)]

[Illustration: A COCONUT GROVE IN THE PHILIPPINE ISLANDS. The people of

tropical regions have more uses for this plant than there are days of

the year. Its fruits will float in the sea for months without injury and

it is thought to have been distributed all over the tropical world by

ocean currents. Its true wild home is not certainly known, but is

probably tropical America. See chapter V for an account of the tree.

(_Courtesy of Brooklyn Botanic Garden._)]

dressings were made, and the collection of sphagnum from the bogs in

which it nearly always grows was the task of many who could render no

other service.

The reproduction of sphagnum is not unlike that of ferns already

described. There is the same necessity of a film of water in which the

free swimming male can fertilize the female. But some other things about

their reproduction of young differ from ferns.

In the first place sphagnum is a nonvascular cryptogam, in that its

leaves have no veins or ducts in them and its minute stem is also

without those conducting passages that characterize all ferns, and the

flowering plants, which are considered the most highly developed of all

plant life. (See Chapter I for a discussion of this point, in the

section devoted to “Flowerless Plants.”)

In this moss, also, there are small branches, some of which bear only

the tiny leaves, but some bear leaves and the reproductive organs. The

female or _archegonia_ are much like those in the ferns, and the

_antheridia_ or male are also, as in the ferns, minute globular organs

in which are the male cells. The branches bearing males are greenish,

yellow, or even reddish, quite unlike the ashy gray foliage leaves which

give to sphagnum its characteristic ashy gray color. Unlike the ferns,

the male cells of sphagnum have only two tails, but they nevertheless

swim, tail first, to the female, when the time for fertilization comes.

The female branches are found mostly toward the upper end of the plant

and bear the archegonia at their extremities.

From what we know of the reproductive stages in the ferns it is now

obvious enough that in sphagnum moss, as we ordinarily see it, we have,

because it bears antheridia and archegonia, a quite different condition

from the ordinary spore-bearing leaves of ferns. For as yet spores have

not been developed on the moss. The mating of male and female cells,

directly on the plant, proves that in this “plant,” at least, our

ordinary notion of this moss is mostly confined to a stage in its life

history comparable in ferns to the production of archegonia and

antheridia on the fern prothallus. From this mating of the male and

female cells there results, as in the ferns, the production of a

spore-bearing structure. This consists of a spore case, matured for the

most part in the chamber occupied by the fertilized female cell, but

ultimately its cap is carried upward. Later on the spore case ruptures,

releasing the spores. As in the ferns, these germinate, forming a short

green protonema followed by a prothallus. From this a short leafy branch

develops, which completes the life cycle, as this is the young moss

plant.

In other words, sphagnum, as we ordinarily see it, produces, on the

plant, male and female cells which unite to form a spore case with

spores in it. These are shed, develop into a protonema which is followed

by the prothallus and from this the young moss plant develops. In ferns

the conspicuous well-known stage is the spore-bearing one, in sphagnum

it is the production of male and female cells directly on what appears

to be the mature plant.

There are many other kinds of mosses than sphagnum, and their life

histories differ in slight degrees from it. But they all agree in this,

that the greenish, feathery little moss plant is a stage in its life

history bearing male and female cells, the mating of which produces a

spore-bearing contrivance. In most of the familiar green mosses this is

a capsulelike body on a short stalk, usually well elevated above the

green mass of plants. From this the spores are shed and develop into a

protonema or “first thread” just as in ferns. Unlike them, and unlike

sphagnum, the green mosses produce no thallus, and the young leaves of

the moss are developed directly from this protonema.

LIFE HISTORY OF A MUSHROOM

The common mushroom that we eat is easily enough divided into a thick

stalk, known as a _stipe_, and a broad hood called a _pileus_. The under

side of the pileus is seen to be composed of thin plaits set closely

together and radiating from the center toward the edge. These are known

as _gills_. From among the gills the spores are shed when they are

mature, usually foretold by the changing of the color of the gills from

whitish to purplish and even to brown or blackish. The spores are then

shed and ready for the next stage. From what we already know about ferns

and mosses, it is clear that from these spores a mushroom cannot develop

without the production of male and female cells and all the rest of that

process of hidden marriage that characterizes all flowerless plants. But

in most mushrooms no one has ever seen, nor have the most carefully

conducted experiments ever demonstrated the germination of the spore. So

far as we know at the present, many mushrooms may or may not produce

their young through the germination of their spores in their native

fields and meadows and the subsequent production of male and female

reproductive organs. But if their spores do produce such organs, which

all our knowledge of spores makes probable, it is, in a truer sense than

in most cryptogams, a case of hidden marriage. The process of producing

their young is thus a secret one that scientists have not yet been able

to disclose. Of course it is a common practice of mushroom growers to

purchase _spawn_ from seedsmen which under favorable conditions will

produce many young mushroom plants. This, however, is the production of

young without mating of the sexes, a fairly common characteristic of

many other plants which will be considered presently.

As we saw in the section devoted to Flowerless Plants in Chapter I,

there are many other kinds of fungi than the familiar edible mushroom

and their close relatives, the often deadly poisonous toadstools. The

reproductive processes in these other fungi are fairly well understood,

but they can hardly be included here. In the mold on bread, the yeast

used in baking, the rust of wheat and the diseases of other plants and

of animals, the individual organism is so minute that it can only be

detected under the microscope. Their reproductive processes are, of

course, on such a minute scale that they could be followed with profit

only by those equipped to study them. They have been described in many

botanical textbooks, and those interested in them should consult such

books.

In recapitulating the reproductive processes in cryptogamous plants the

thing that distinguishes them from all flowering plants is that they

bear, in some stage of their life history, a spore. From this, in the

great bulk of them, a mature plant never develops. Only by the

production from the spore of some contrivance for bearing male and

female cells, which may, as in some seaweeds, even be on different

plants, can a mating of these be accomplished, and from this union will

develop the mature plant. There are many modifications of this plan, but

in nearly all of them the presence of water, for the free swimming of

the male cell to its mate, is essential. Just as in flowering plants and

in all the larger animals, however, the reproduction of young in

cryptogams is a sexual process depending on the union of male and

female. While in phanerogams that process may well be spoken of as

visible marriage, with all the pageantry of insects and beautifully

colored flowers, in cryptogams the process is not only a hidden

marriage, its ways are sometimes so secret that, even in the common

mushroom, the actual mating is conjectured rather than demonstrated.

THE PRODUCTION OF YOUNG PLANTS WITHOUT MATING

It is so generally true in all plants that a union of male and female is

necessary for the production of young, and, as we have seen in most of

them, the process is so uniformly successful that still another mode of

producing them seems almost unnecessary. Yet in a surprisingly large

number of plants new individuals, both of flowering and flowerless

plants, are regularly produced without such a union and where sexuality

has nothing to do with the increase.

In the life plant--a thick-leaved shrub from Mexico commonly grown in

greenhouses--the leaves are wavy margined. From their edges, especially

when injured, many tiny new plants will often start to grow. Even if the

leaf is cut up into fairly small pieces many of these will develop young

plants, and in various forms of the common rex begonia the leaves are

usually cut into small pieces by gardeners for the production of young

plants which always sprout from such pieces. It is useless to multiply

such cases, as everyone knows of the production of young plants from the

ends of strawberry runners, the cutting up of potatoes, the universal

garden practice of making cuttings, and the sprouting of willows, all of

which are effective by virtue of this faculty of plants to produce young

quite without the intervention of different sexes. Not so well known are

the cases of a liverwort, a small relative of the mosses, which, if

chopped into fine pieces, each will develop into a new plant. We have

already spoken of the spawn of mushrooms; and even on sphagnum moss, in

addition to its sexual reproduction, it produces sterile branches that

will root and, after separation from the old plant, form a new one.

Wherever this tendency is found, whether it be in a microscopic seaweed,

some of which know no other means of reproduction, or in the showy

begonia, it depends for its success upon a property of the ultimate unit

of its structure, the cell. Sometimes, as in bacteria or the most minute

seaweeds and in some other kinds, the whole plant consists of a single

microscopic cell, when it is said to be a _unicellular_ plant. All

others, in which the grouping or modifications of the cell makes more

complex structures, such as trees or shrubs and all the plants that

grow, both flowering and flowerless, are called _multicellular_ plants.

Whether they be of one or many cells, these have the faculty of

dividing, and by this division making two where one existed before the

division. This division of cells is what happens in the normal growth of

plants and it is this division, in more unusual ways, that results in

the production of new plants without mating of the sexes. As cells are

themselves microscopic, of course their division is equally so, and

cannot be described in detail here. It has been many times described and

pictured both in books on plants and animals, as it is the ultimate unit

of the structure of both.

Plant life, then, seems to be better provided with means to renew itself

than most animals, for, as we have seen, it has several methods to rely

on. These may be divided into sexual, which includes both that in

flowering plants with their visible mating and in flowerless plants with

invisible mating, and _asexual_, literally without sex. In the latter

are all those unicellular plants that reproduce themselves by simple

division of the cell, and also those flowering plants that either

naturally, as in life plant, or by the gardener’s art of making

cuttings, produce new plants quite without the intervention of the

sexes. Whether it be sexual or asexual, nature has more than fulfilled

its obligation to the plant world in providing it opportunities for

self-renewal. No matter what apparently unfavorable condition arises and

often in spite of an almost unbelievable wastage of potential life

stuff, the renewal goes on, or else there is the total disappearance of

the species. So strong is this tendency to provide for renewal of their

kind that many plants, if injured or cut by a mower, will almost in

their last gasp hurriedly flower and set seeds, and we have already seen

that the little liverwort, even if cut to pieces, also obeys that nearly

universal law of nature: “Be fruitful and multiply.”