The Life of the Plant

plants we can still more easily be convinced of the fusion of the substance of male and female cells. The nature of this phenomenon, the chemistry of the process, is almost unknown to us. A ferment has been dis- covered among the constituents of the pollen grain, and moreover during pollination increased chemical activity is known to be generally displayed by flowers : they greedily absorb oxygen and give off carbonic acid, and this respiration is accompanied by a perceptible rise in temperature of the whole flower, and especially of the stamens.

However, the result of this fusion manifests itself in the awakening of formative activity in the embryonic cell. It surrounds itself with a cellulose wall, becomes divided by a partition, thus becoming two cells instead of one (fig. 65, at the top, on the left). The first parti- tion is followed by another, then by a third, and so on, till a multicellular body is formed, which grows and develops into the seedling which we met with in our first lecture, in talking of the seed. Sometimes a single seed has several embryos. This does not often happen. There are several embryos in the seeds of an orange, for instance, but they have quite a peculiar origin, and it would be useless to enter here upon a description of the process. Throughout the develop- ment of the embryo stores of nutrient substances are formed in other parts of the ovule, in the embryo-sac, and in the nucellus ; this will be the part of the seed which we have called the endosperm. 3 The endo- sperm being a store of food, it may either be absorbed by the embryo while the seed is still on the maternal plant, in which case the ripe seed will not contain any endosperm, like the seeds of beans ; or, on the other hand, most of the endosperm may be preserved in the ripe seed, as in cereals, buck-wheat, the poppy, etc.

development of the embryo, and the transformation of the ovule into the seed ; its influence also extends to the carpel, which grows after fertilisation and develops into the fruit. Such in very general outline is the formal morpho- logical aspect — the only one known to us — of this phenomenon of fertilisation. Let us now observe how the same process takes place at the other pole of the vegetable kingdom, among the simplest plants, like the weeds and the moulds.

Here is a microscopic weed, Spirogyra, which we have already studied, with its characteristic spiral bands of chlorophyll (fig. 66, right-hand side). At a certain stage of development the filaments, of which the organism consists, become parallel to each other, as is shown in the plate. In some cells the contents gather into lumps, round or oblong in shape. Meanwhile the walls of two adjacent cells form swellings, which grow towards each other and meet ; the partition dividing them is absorbed and vanishes, and the contents of the two cells fuse, the contents of the right-hand cell

flowing into the left, or vice versa. The spherical mass thus produced surrounds itself with a wall, and becomes a spore. When set free this spore is capable of germina- tion, and will give rise to a new organism, a new fila- ment of Spirogyra. Here, then, we meet with an extremely simple process of fertilisation : two cells fuse in order to form a new cell which serves the purpose of reproducing the organism. This phenomenon, in a still simpler form, is met with in a microscopic fungus — a mould. This mould con- sists of a thin very much branched tube (fig. 66, left- hand side) in which no transverse partition is to be seen ; which means that the whole organism consists of a single cell. In some parts of this cell short branches appear, stretching towards each other. Whenever they meet, the ends of the branches are separated from the rest of the plant by partitions, and swell. Later on the wall disappears where the ends meet, and the contents fuse, thus forming a single cell — the spore.

Thus in the simplest spore-bearing plants, as well as in the flowering plants, the phenomenon of fertilisa- tion consists in the fusion of the contents of two cells. This phenomenon is even more obvious in spore- bearing plants, because we can actually observe there the fusion of two cells. In the simplest cases we have described, no difference has been noticed in the aspect of the male and female cells, although a closer study of their structure does reveal a distinction between them ; but in other more complicated spore-bearing plants the male cell differs entirely from the female cell in form as well as in character. While the female is non-motile, the male moves about like a microscopic animal, and penetrates (by its own activity) into the organ containing the female cell. There it fuses with it, dissolving in it, so to speak, and fertilises it. 1

1 In 1897 such motile cells were found in the pollen-tubes of some seed plants. We can follow with still greater certainty the effect of the pollen upon the ovule by means of experiment. In the first place we know that if the stigma is not pollinated the flower will fade without producing any seed or fruit ; further, when pollen -tubes have been made to act under the microscope upon ovules, removed from the ovary, it has been found that the effects of fertilisation were manifested only when a pollen-tube came into contact with an ovule. Finally, the partici- pation of the male element has been most conclusively proved by experiments in artificial hybridisation. If the pistil of a flower is pollinated from another flower, differing from the first, say in the colouring of the petals, it is possible in some cases to produce a plant with variegated flowers, i.e, flowers which have both the colour of the petals of the one in which the pistil was fertilised, and the colour of the other the pollen of which was used. Obviously the effect of the male cell has manifested itself in the plant which resulted from the process of fertilisation.

In order to accomplish the requisite pollination, plants are provided with a number of various adapta- tions. Let us dwell on some of them. Here is a plant (Piled) rather insignificant in appearance, but grown in hot-houses on account of the following curious pro- perty : whenever the flowering plant, covered with unsightly little flowers, is sprinkled with water, small clouds of dust rise here and there from its surface as if from minute explosions. This phenomenon is due to the stamens of this plant (coiled inside the flower and very hygroscopic) suddenly uncoiling like springs, and shedding the pollen out of their broken pollen- sacs. The pollen scattered in this way easily reaches the stigma. Let us study another case, this time a plant living in quite a different medium — a water plant. All lovers of indoor aquaria are familiar with their most common inhabitant, Vallisneria . The

stamens and pistils of Vallisneria are found on different flowers, and these are distributed on different plants water. Gardeners know, for instance, that rain during the flowering period interferes with fertilisation. Under these conditions the flowers remain sterile and produce no fruit. In order that the fertilisation of Vallisneria may take place in the air the plant is provided with the following ingenious adaptation. The female flowers (left-hand side) grow almost at the bottom of the water, on very long but tightly coiled stalks. When the flowering season approaches these stalks uncoil and grow, thus bringing the female flowers up to the surface of the water. By this time the male flowers which develop similarly at the bottom of the water (right-hand side) are torn from their stalks and also rise to the surface of the water. Floating among the female flowers they open their anthers and shed their pollen, some of which falls on the stigmas of the female flowers. When the flowering period is over, the stalk of the female flower coils up again, carrying the fertilised flower down to the bottom of the water, where the further development of the fruit takes place.

The significance in a plant’s life of the pollen and the ovule — the essential parts of the flower — is now quite clear. The adaptations described above which make for their mutual interaction are also comprehensible. But another question springs up: what is the significance of the remaining parts of the flower ? What is the pur- pose of the calyx ? What is the use of the carpel which only hinders the access of pollen to the ovules ? Why have the petals such bright colours and some- times such fantastic shapes ? What is the purpose of the perfume of flowers, and, finally, of the sweet honey- like fluid secreted at the bottom of the corolla by the well-known clover, dead-nettle, and many other flowers ? Let us try to answer these questions. The significance of the calyx and the carpel is the most intelligible of them all. The former, like the external scales of leaf-

buds, guards the inner more delicate organs of the flower during their development ; the latter plays the same part towards the ovules enclosed within it. In the experiments on the fertilisation of ovules detached from the ovary, that we have just described, the experimenter met considerable difficulties in struggling with minute parasitic organisms, bacteria, which have so terrible a reputation owing to the infectious diseases they produce. When we wish to preserve organic matter from decay, we must protect it from becoming infected with bacteria. This we do by keeping it in hermetically closed vessels, or at least guarding it from the access of the germs of these organisms, floating about in the air, by closing the necks of the vessels with cotton wool. The cavity of the ovary is exactly the kind of vessel in which the ovule and the seed may safely develop without coming into contact with the germs of parasitic bacteria floating in the air. But an objection may be raised here : if on reaching the surface of the stigma the pollen grains germinate and their pollen-tubes reach the ovule, why should not bacteria flying about in the air likewise develop on the stigma and reach the ovule in the same way ? The same observer to whom we are indebted for the explanation just brought forward of the significance of the ovary, has also explained the special significance of the stigma. Pollen grains developing outside the flower (under the microscope) suffer from bacteria in the same way as ovules. In order to prevent their attacks he slightly acidified the liquid" in which the pollen grains germinated. This slightly acid reaction, which was harmless to the pollen grains, did actually prevent the development of bacteria. On testing the stigma itself, he found it also had an acid reaction.

Thus, while it lets the pollen-tubes pass through, the stigma apparently prevents the access of bacteria to the ovary. But how do the bright petals, the perfume, and the •honey glands serve the flower ? At first sight their function seems to be purely aesthetic. Formerly, when man considered himself the centre of the universe, when even the sun was believed to revolve around him, it was easy enough to admit that everything existed for the benefit of his eye, his sense of smell, or his taste. But gradually, with the development of science, this point of view lost more and more its hold. On the other hand, failing to observe in these parts of the flower any immediate purpose, botanists were in the habit of calling them non-essential. They looked upon them simply as the plant’s bridal dress, to use a poetical and meta- phorical expression. Both opinions proved to be wrong. To begin with, all these organs turned out to exist not at all for man, but for insects, and, above all, for the plant itself ; secondly, they proved to be very essential, something even absolutely necessary for plants ; without them the so-called essential organs themselves would not have answered their purpose ; and, finally, they were proved to be useful precisely in so far as they are bright, perfumed, and sweet, i.e. in so far as they serve as bait for insects. Let us explain the matter.

The ecclesiastical and civil laws of most nations, except those at the lowest stage of civilisation, censure, forbid, and even persecute marriages between near relations. Doctors and physiologists have tried to prove by statis- tics the justice of this law so widespread over the earth, and indeed there is abundant proof that near relationship between parents has a very bad effect on their children’s health. To-day such proofs are scarcely needed any longer, because a whole series of investigations prove that it not only applies to man and the animal kingdom, but also to the vegetable kingdom ; that this law is general for all the organic world. Nothing so convinces us of this truth as the plant.

by pollen from the same flower is less successful, and gives a less vigorous posterity than when the pollen comes from another flower. Moreover, there are plants in which self-pollination is absolutely fruitless, e.g. Cory- dalis. There are also plants with two or even three kinds of flowers, e.g. the primrose, the purple loosestrife, and the different species of flax. The pistils and stamens of these flowers are of different lengths, and moreover long pistils occur in the same flower with short stamens (fig. 68 A) and vice verst (B). For successful fertilisa- tion, it is necessary to transfer pollen to the stigma from

stamens corresponding in height, which means that the pollen must always be taken from another flower. 1 There are plants where fertilisation with the pollen of another variety turns out to be more fruitful than self- fertilisation. Finally, quite trustworthy investigators have described cases in which self-fertilisation acts like poison ; the stigma of a self-pollinated pistil appears as if singed, and the flower dies without forming fruit, whereas pollen taken from other flowers effects fertilisa- tion. Thus a number of facts prove empirically the existence of a general law, according to which cross- fertilisation is beneficial and self-fertilisation compara- tively harmful to the organism.

1 In the middle of fig. 68 the pollen grains, which are of different sizes, are shown. The existence of this law once admitted, the signifi- cance of a number of peculiarities in the structure of the flower, otherwise inexplicable, becomes clear. A great number of facts have been accumulated in botanical literature in support of the theory that the structure of flowers is adapted to cross-fertilisa- tion, especially through the agency of insects. Let us here dwell only on the most prominent outlines of this theory.

The following considerations can be brought forward in support of the theory that a showy appearance, perfume, and honey glands exist for the purpose of attracting insects, which in flying from one flower to the other promote cross-fertilisation. In the first place, all these characters exist only during the period of pollination, after which they disappear. Secondly, there is a marked difference between plants in which pollination is effected by means of wind and those in which it is effected through the agency of insects. Flowers of the former kind are generally small and unsightly, are never highly coloured, and possess neither perfume nor honey. Such are the majority of trees, e.g. the pine, the birch, the aspen, and so forth. On the other hand, these plants produce an abundance of pollen. Clouds of yellow dust falling on the soil or on the surface of water have even given rise to superstitious beliefs in rains of sul- phur. It is noteworthy that such flowers generally bloom in early spring, when the undeveloped leaves cannot prevent the scattering of the pollen.

It is clear, nevertheless, that such a waste of material probably precious to the plant, a waste inevitable during pollination by the wind, must be a very great disad- vantage to the plant. Apparently it is more profitable to a plant to produce less pollen and to secure more accurately its transference to the stigmas of other plants. This economical purpose is evidently served by insects. Honey glands, the so-called nectaries, entice insects to visit flowers ; the bright colouring, the size and the perfume of flowers serve as means for attracting them. It has been proved that bees possess the faculty for distinguishing between colours. By smearing with honey differently coloured surfaces, we can train insects to associ- ate the impression of a certain colour with the presence of honey. Cross-fertilisation takes place as a result of a division of sexes in the flowers themselves ; the staminate and pistillate flowers may be distributed either on one and the same plant or on different plants : while in hermaphrodite flowers cross-fertilisation is achieved by the ripening of stamens and pistils at different times, whereby self-fertilisation becomes impossible. But most convincing of all are the numerous facts which show all the parts of the flower to be adapted in structure and disposition to the form and habits of the visiting insect, so that in flying from one flower to another the insect is

bound to touch the stamen of one flower, and then with the same part of its body the stigma of an- other. Let us limit ourselves to a few illustrations. There are many flowers the parts of which, instead of being arranged in the usual regular way round the centre, are distributed in such a way that right and Fig. 69. left sides, top and bottom can be dis- tinguished. Labiate flowers may serve as an example (fig. 69, 2) Such a form is obviously very convenient

for insects ; the lower lip serves as a platform or balcony, on which the insect alights when in the act of passing its proboscis into the tube of the corolla at the bottom of which the sugary nectar lies. But in doing so it rubs its back against the an- thers (fig. 69, 1 and 2 m), and when it alights upon another flower it rubs its back covered with pollen against the stigma (p). Self-pollination is quite im- possible in this case, because the stigma develops later than the stamens of the same flower, and it can receive pollen only from another flower. In another labiate flower, the common sage, the following curious adaptation is found (the sage flower is shown in fig. 69). The stamens are of an unusual form, having the anthers fixed to the top of a long cross-piece, which is balanced on a short stalk (in fig. 69, x, a flower is shown split longitudinally, with one stamen ( m ) ; in fig. 69, 3 the two stamens are shown separately) . The lower ends of these cross-pieces bar the entrance to the tube of the corolla, at the bottom of which, as has been already said, there lies some sugary liquid. The moment the insect pushes its proboscis into the tube, it sets in motion these lower ends of the cross-pieces, which like levers come into position 4 from position 3 (fig. 69) . At the same time the anthers descend on the back of the insect (fig. 69, 2 m) and cover it with pollen.

Another still more striking method of pollination is found among the orchids, where in most cases pollination would be almost impossible without the agency of insects, and the flowers would be doomed to continual sterility. The flowers of this family are distinguished by their fan- tastic shapes, for which they are so commonly grown in hot-houses. It may be sufficient to mention among the wild representatives of the group the butterfly orchis (Platanthera ) . Let us imagine such a flower with all its petals torn off, with the exception of the lowest which is in the shape of a lip and is elongated at the base into

a tube, called the spur (fig. 70 sp.). 1 The pistil and stamens of this flower are equally singular in shape : the pistil consists of a long twisted ovary ( ov .), on the top of which, without any style whatever, the stigma rests, in the shape of a sticky spot at the very entrance into the tube of the spur (stg.) . The stamen has no filament, but consists of an anther only ( anth .) , situated close to the stigma. But all this does not ex- haust the pecu- liarities of this plant. Its pollen is not crumbling dust, but is gathered in each of the two lobes of the anther into a lump, on a stalk which ends at the bottom in a sticky knob on the out- side of the anther (fig. 70, 2). Evidently since the pollen is not set free by itself it cannot reach without external assistance even the stigma of the same flower, although it lies so close to it. This very assistance is given by the insect. In settling upon the lip (il), it thrusts its proboscis into the tube of the spur, at the bottom of which a sugary fluid is generally secreted, distinctly seen in Platcmthera. The insect invariably comes up against the viscous knob sticking out of the stamen, and in flying off the flower carries away its mass of pollen. This arrangement of the flower is so accurate, and works so perfectly, that even a needle cannot be thrust into the spur in the direction of

1 Fig- 70 shows the flower of an orchid, all the petals of which are removed except the lip, which is split into two, to show the entrance into the spur and the position of the stigma. the arrow (fig. 70, 1) without being removed with the mass of pollen adhering to it, as is shown in fig. 70, 3. At first this mass of pollen stands upright, but in a few minutes it bends forward (fig. 70, 3). The same thing happens to the pollen mass adhering to the insect. When it flies off to another flower, where the same process is repeated, it leans with the pollen mass exactly against the viscous stigmatic surface ( stg .) and leaves there part of the pollen. The fact that everything actually takes place in the way described above can be proved from insects caught during their visitation of these flowers. On their proboscis and head masses of pollen have been often found adhering in considerable numbers. Conse- quently, the fertilisation of such flowers cannot take place without the participation of insects and resolves itself into cross-fertilisation, always an advantage to the plant. The importance to the plant that its floral parts should have just these and not other forms becomes quite apparent.

Let us consider another striking example where all the parts of the flower appear to be adapted to the same end, i.e. to cross-fertilisation by means of insects. This plant is Aristolochia. Its pale yellow flower has a tubular corolla swollen into a ball at the base, expanded and cut obliquely at the apex. The long ovary with a number of ovules (fig. 74) passes immediately without any style into a lobed stigma. Stamens without any filaments and closely attached to the pistil are situated right down under the stigma. The narrow tube of the corolla is lined with stiff hairs pointing towards the inside of the flower, as is shown on the plate (71, upper section). The whole arrangement is like a mouse-trap. Owing to this position of the hairs the insect easily crawls inside the flower, but cannot come out again ; the flower is a trap where the insect finds a stigma already mature and ready for fertilisation, together with stamens not yet fully developed. If the insect comes from another flower it rubs pollen on the stigma. Some

time later the stamens ripen and split ; the insects restlessly move about in their prison (71, upper section) and get covered with pollen ; but this pol- len on reaching the stigma does not pro- voke self-fertilisa- tion, because the stigma has already withered. The hairs of the corolla soon afterwards wither and fall off (7 1 , lower section) , the door of the prison is open, and the insect, covered with pollen, flies out to be caught again by a similar trap in another flower. Later on,- the upper lobe of the corolla withers, and bending over closes the entrance into the flower (71, lower sec- tion) ; and the whole flower, until then erect, droops. In this way fertilised flowers escape useless visits from insects. This adaptation works as successfully as the mechanism for the fertilisa- tion of the orchids, so that we can tell almost with certainty from the external appearance of the flower whether we shall find insects inside it or not. The number of insects thus temporarily imprisoned in a flower is sometimes considerable.

We may, then, assuredly see in the so-called unessential parts of the flower, such as the bright colouring of the corolla, its fantastic shapes and honey glands, very perfect adaptations of the flower to the purposes already indicated. Perfume probably has a like purpose. Like colour it serves to attract insects. It has even been noticed that flowers, which exhale in the night a very strong perfume, are visited by night insects. Yet the exhalation of volatile, aromatic substances can serve another purpose in the plant. To begin with, plants need a more or less high temperature in order to flower, which is partly supplied to them by the above-mentioned rise in temperature as the result of respiration ; but, on the other hand, during clear, calm nights plants are known to be exposed to considerable cooling owing to strong irradiation. To prevent this cooling we have only to cover the plant with a glass bell which arrests the heat rays emitted by the plant, and thereby prevents unnecessary cooling. But this very property of the glass belongs also to all volatile substances such as are exhaled by flowers ; they strongly arrest radiant heat. In order to ascertain how considerable is the atmosphere of the volatile, so-called ethereal oils, surrounding scented plants, we have only to apply a lighted match to the strongly scented flower of Dictanmus. We see in amaze- ment one flower and then the whole bush of flowers enveloped in a bluish flame. This means that the vapours of ethereal oils excreted by glands with which the flower organs of this plant are supplied have become ignited. Therefore, during calm, clear, summer nights, i.e. exactly when there is a danger of cooling owing to irradiation, the flowers are surrounded with a trans- parent cloud, an atmosphere of these emanations, which, by arresting like the glass bell the heat radiating from the flowers, preserve them from excessive cooling . 1

1 Plants are known to be preserved from cooling in the night by the lighting of a fire in their neighbourhood, which will make a cloud round them and prevent them from losing too much heat by irradiation. many that science has been causing the pride of man from the moment it proved that it is not the sun that revolves round him, but he round the sun — namely, that this variegated carpet of flowers, glowing with all the colours of the rainbow and emitting the most delicate perfumes, has never existed for his sake — the king of Nature — but for midges and insects, and, above all, for the sake of the plant itself.

But with every new achievement of science, although it has involved his parting with unjustified claims, man has only been gaining in real importance. So is it in this case : if he has to admit that flowers were not created for his sake, he can surely console himself with the thought that they are partly created by him. We have only to compare the plants in our flower gardens, kitchen gardens, and cultivated fields with wild plants to agree with this statement. A glance at any cultivated plant, at any garden flower, or any vegetable reveals in them the guiding hand and thought of man. His claims, sometimes even only a passing whim of fashion, have changed natural things in accordance with those claims and whims. Fancy demands, for instance, that a small irregular, pale, three-coloured flower of heart’s-ease should become big, of one colour, almost black, and round ; and here, before our very eyes, as if by some magic power, we see it actually becoming bigger, darker, and rounder. The question arises, by what means has man attained this result ? how has he obliged Nature to contribute to his ends, to follow obediently his indications ?

The procedure is very simple : man has been working on these lines for ever so long without realising it ; and it is only recently, after having grasped the treasures of knowledge accumulated during centuries of practice, that science has presented in their true light the sim- plicity and universality of the method employed. This procedure is as follows. Seeds obtained from one and the same plant never produce two absolutely similar plants ; differences invariably appear. If these plants be left to grow and reproduce themselves, their differences will soon vanish ow T ing to cross-fertilisation, and a permanent, mean, typical form will be produced. Quite a different result will be obtained if a form distinct from others in some point or other be removed and isolated ; its peculiarities will in most cases pass to its posterity. If in the new generation we again isolate the specimens which have attracted our attention by a striking peculiarity, we shall emphasise the peculiarity in every generation, and eventually fix it. This is the method of selection.

In horticulture this method of selection is often applied in the simplest and at the same time most effective way. It consists in the destruction in every generation of all the plants that fail to answer the purpose in view. By repeating this operation of selec- tion in every subsequent generation, and so strengthen- ing minute and scarcely noticeable characters of the plant, man seems to cast it in a new mould, feature by feature, and to realise an anticipated ideal. It is worth noticing that, while thus breeding plants and animals, man has applied this principle of selection to himself as well, although unhappily only in the reverse order. For ages he has generally chosen the best representatives of his own kind from the physical point of view and condemned them to certain death. This experiment with humanity proves, though of course in the negative direction, the success of the principle of selection. Such was, for instance, one of the results of Napoleon’s work. His endless wars have resulted in the decrease of the average stature in France.