Popular Science Monthly/Volume 39/July 1891/Pollen: Its Development and Use
|POLLEN: ITS DEVELOPMENT AND USE.|
By JOSEPH F. JAMES, M. Sc.
WHEN a plant in growing has reached a certain stage in its development, the character of the buds which have before produced branches changes, and flower buds appear. In correspondence with the change of character in the buds, the stem and leaves change also. The former becomes smaller and forms the peduncle of the flower, while the latter, dwindling from true leaves or foliage organs, become bracts, sepals, and petals.
The process of flowering is attended with very important results. While the plant has been growing rapidly, sending out new leaves and branches into the air and new roots into the earth, the total amount of material produced by general growth has not been expended. A certain portion is kept in reserve, and this is drawn upon when the time for flowering and fruiting has come. It is the exhaustion of this reserve of food which causes annuals to perish after perfecting their seed. In biennials a store of matter is laid up one year in the leaves or roots, to be drawn upon by the plant when the flowering time comes round the next year.
Lamarck, about seventy years ago, was the first to detect that a certain amount of heat was evolved on the expansion of the flowers of the European Arum. Their anthesis is equivalent to a burning up of some of the material of the plant. The heat so produced is sometimes quite considerable, and it can be measured by means of a thermo-electrical pile. A most notable example of the consumption of stored material is to be observed in the century plant. This grows for many years, laying up nourishment in its large, succulent leaves. After from fifteen to seventy years' growth the time for flowering comes; the reservoir of nourishment is drawn upon, the flower stalk is shot up with tremendous rapidity, and in a few weeks the thousand blossoms have opened, faded, and seeded. Then the whole plant dies. It has exhausted its store of nourishment, and consumed itself in the production of seed. Dr. Gray has likened the plant to the fabled phoenix, which, consuming itself in giving birth to its offspring, literally rises from its ashes.
All the parts of a complete and perfect flower are, morphologically speaking, modified leaves. It can often be observed that true leaves pass insensibly into bracts. These in turn pass into sepals, and these again into petals. Sepals, being as a rule green, can be more easily seen to be modified leaves than petals. The last are usually colored, and the fact is not so noticeable. Yet, in the flowers of the cacti, the line between the outer bracts and the sepals, and between these and the petals, can not be drawn, for they pass imperceptibly into each other. In the Nymphæa (water-lily) there are numerous rows of petals, and a gradual change can be traced from the outer row of petals into stamens. First at the tip of a petal are developed two small lobes, one on each side. These lobes enlarge as the center of the flower is reached, and at last a fully formed anther at the top of a slender filament is the result. All the various stages through which the stamen has passed are visible in the rows of petals.
According to Mr. Grant Allen, the original and primitive flowers were made up of stamens and pistils only as the essential organs of the flower; and the petals and sepals are but stamens modified by insect agency. Now, whether petals are regarded as modified stamens or stamens as modified petals is immaterial. There is nothing to prevent the adoption of both views. The stamens must, in the first place, have been leaves; for it often happens that ordinary leaves are found bearing pollen grains on their edges. So, too, the anther is to be regarded as the modified apex of a rolled-up leaf. As the flowers became, in the course of time, more and more suited to insects, some of the stamens were doubtless changed back into leaves in the shape of petals and sepals, while at the same time the true leaves of the stem may have been changed into bracts of various sorts.
There will be noticed, on the examination of any ordinary stamen, two principal parts. One is the long, slender stalk or filament, and the other is the knob at the end, or the anther. The filament, says Sachs, is to be regarded as the staminal leaf. The anther is made up of two lobes, situated at or near the apex of the filament, one on each side, and separated by a prolongation of the filament known as the connectile. In these two lobes the pollen is developed. Sachs says that "the formation of . . . the pollen grains of phanerogams always takes place by the division of the mother-cell into four parts." This division takes place as follows: In the process of growth of the original mother-cell, the nucleus becomes divided into two parts, each soon forming the center of a new cell. These two again each divide and finally form four, each one surrounded by a cell-wall, but all still inclosed by the wall of the mother-cell (Fig. 1). On further growth the wall of the mother-cell is ruptured, and the daughter-cells escape as free pollen grains. The wall of the mother-cell is then either absorbed, or remains in the form of threads between the grains, or as viscid matter on the outside of the grains.
Thus pollen grains are daughter-cells, which have been developed from a primal mother-cell. Each grain is made up of three parts. There is an outer wall (extine), an inner wall (intine), and the fluid contents (fovilla). The extine is often marked with lines, points, or grooves; the intine is generally smooth and regular, and, even when the extine is studded with points, the intine does not
|Showing marks on extine.With extine removed.|
|Fig. 2.—Hollyhock||Fig. 3.—Pollen of Œnothera.|
line the inside of these points but extends over their bases (Fig. 2). In cases where there are projections at different points, as in the evening primrose (Œnothera, Fig. 3) and others, the intine becomes thickened, and the extine is very much thinner. In the melon (Fig. 4), where the extine has pores provided with lids, the intine at these points is considerably thickened, and in growth pushes the cap off. When, again, a pollen grain appears marked with reticulations and spaces, as in Pancratium (Fig. 5), these are regarded as thinner places in the extine rather than special markings. It is a well-known fact that the pollen of anemophilous or wind-fertilized plants differs markedly from that of entomophilous or insect-fertilized plants. In the former case it is dry and powdery, probably having this quality from the entire absorption of the wall of the mother-cell, thus leaving the grains separate in the anther. In entomophilous plants, on the contrary, the pollen is often viscid, or else studded with points, so that it may in some way adhere to the legs, bodies, or probosces of insect visitors.
While, as a rule, the pollen grains are free in the anther cells, there are two families, those of the Orchids and the Milkweeds, in which the grains are developed in a peculiar manner. In these families, but especially in the former, the grains cohere to one another by means of a viscid matter, and thus form one mass, technically known as a pollinium (Fig. 6). The cohesion is brought about by the walls of the mother-cells remaining, and so binding the grains more or less completely together. The cohesion is but slight, for, on the application of a pollinium to the viscid surface of a stigma, several grains are left on it, and thus one pollen mass will serve to fertilize several flowers. One of the most striking features of these pollinia is that the stem or caudicle undergoes on exposure to the air an act of depression (Fig. 7), so that while it stands erect on its first withdrawal from the anther cell, in a short time a contraction in the substance of the caudicle is observed, and then the pollinium becomes horizontal.
While in ordinary cases pollen is yellow, there are instances in which this feature varies. For example, in one form of flower of loose-strife (Lythrum salicaria) it is green. In the willow-herb (Epilobium angustifolium) it is blue. In the tulip it is black, and in mullein (Verbascum) it is red.
The seeds of flowering plants are produced by the action of the pollen on the stigma. Though both bulb and seed bear in themselves the potentiality of the future plant, the two are very different. This difference can be well stated by saying that the bulb perpetuates the individual, and the seed the species. When the anther is nearly ripe, its inner wall becomes thinner and thinner, either along certain lines or in particular spots. The process continues until by pressure the wall is ruptured at these places and the pollen escapes. When the grains are placed upon the viscid stigma, the moisture absorbed by endosmose causes the contents to swell, and the intine bursts through the thin places in the extine and protrudes in the form of a tube, which penetrates the stigma to the ovary (Fig. 8). While the tube is at first only a projection of the intine, it afterward becomes a growth, for it is many times larger than could be contained in the grain. It is, therefore, nourished by the conducting tissue of the stigma. The fovilla, or the mucilaginous fluid filling the grain, proceeds by endosmose into the tube, and thence to the ovule. An effort has been made to disprove the statement that pollen tubes penetrate the style, and so fertilize the ovules; but the great mass of evidence, and the statements of many observers who have seen the tubes in contact with the ovules, indicate that the tubes sent out from the pollen grains do penetrate the style, and then are brought into contact with and fertilize the ovules.
It is a strange fact that pollen grains are often entirely inoperative on the stigmas of the flowers that produce them. It has been found by many experiments that, when certain flowers are inclosed in nets, and insects thus excluded from them, if the pollen be applied to the stigma of the flower that produces it, the capsules never set seed; but, if the pollen of another flower be applied, then the stigma is fertilized and seed is produced. In the California poppy (Eschscholtzia) there is a remarkable instance of this. Fritz Müller, in Brazil, found it completely sterile with its own pollen. Darwin, in England, found that even with the Brazilian stock of Müller he could get only a few seeds. Thus the sterility appears to depend on other things besides the pollen, the climate, perhaps, having some effect. Sometimes, too, it happens that, if the home pollen grows, and then foreign pollen be applied, this last grows faster and crowds out the first sort.
The immense number of pollen grains produced by a single flower apparently militates against the saying that Nature allows nothing to be formed but what is needful. It seems, indeed, a vast waste of material to have such a multitude of grains when so very few would answer the same purpose. In a single flower of the peony there are about three and a half million grains; a flower of the dandelion is estimated to produce nearly two hundred and fifty thousand; the number of ovules in a flower of the Chinese wistaria has been counted and the number of pollen grains estimated, and it is found that for each ovule there are seven thousand grains. While few fall below the thousands, many rise far above the peony in point of numbers. These are the wind-fertilized flowers, and here Nature must provide for an immense loss of material. Darwin says that "bucketfuls of pollen have been swept off the decks of vessels near the North can shore.... Kerner has seen a lake in the Tyrol so covered with pollen that the water no longer appeared blue.... Mr. Blackley found numerous pollen grains, in one instance twelve hundred, adhering to sticky slides, which were sent up to a height of from five hundred to a thousand feet by means of a kite, and then uncovered by means of a special mechanism." The so-called showers of sulphur which have at times visited various cities, notably St. Louis, are nothing but clouds of yellow pollen blown from pine or other forest trees from some distant place. Perhaps, out of millions of grains thus scattered far and wide, only a single one may be of service.
As if to compensate for this expenditure of pollen in some plants, there are others in which the amount is very limited, and where nearly every grain is made to count. These are known as cleistogamous flowers, a term applied to those which always remain in the bud. These flowers are found in plants belonging to about sixty different genera of various orders, and generally in those species which at the same time produce the normal and conspicuous flowers. These large blossoms are often sterile, and the plant must depend on the cleistogamous flowers for its seed. In the wood-sorrel (Oxalis acetosella), these flowers have each about four hundred pollen grains; the touch-me-not (Impatiens) has only two hundred and fifty, and some violets only one hundred. Even before leaving the anther cells the grains in these cases have protruded their pollen tubes; these seek the pistil and penetrate to the ovules.
It might perhaps be supposed that, as the seed can be produced so easily, all plants would have cleistogamous flowers. But here
|Fig. 10.—Corolla of Kalmia, (After Gray.)||Fig. 11.—Erica tetralix, with Pendent Anthers and Processes. (Alter Lubbock.)|
comes into play the fact that continual close fertilization is a great detriment and not a benefit, and that it is better in the end that flowers produce an apparently wasteful amount of pollen and take the chances of a cross, than to be more economical and be perpetually self-fertilized.
A few words now as to the movements of the stamens in connection with the ejection of the pollen. The contrivances for this are various. In the barberry (Fig. 9) the anthers are provided with valves which fly up and throw out the pollen when the base of the filament is touched. In the sheep-laurel (Kalmia, Fig. 10) the anthers are lodged in little pits on the corolla lobes, and the filaments are in a state of tension. The anthers open by terminal pores, and when the base of the filament is disturbed the anther is released from the pit and flies forward, this movement throwing the pollen out of the pores at the apex. The anthers of the heather (Erica tetralix, Fig. 11) are provided with processes projecting backward and nearly touching the sides of the corolla tube. As they have terminal pores, and are pendent, when the processes are jostled, as they would be by the visits of insects, a shower of pollen falls upon the visitor. Lastly, in the Sage (Salvia, Fig. 12), the anthers are separated by a long connectile which is hinged at the top of the filament in such a way as to cause one of the anthers to come forward and downward when the other one is disturbed and pushed backward. They are at the same time so placed in the corolla tube that the movement is inevitable, and the distribution of the pollen certain, when an insect visits the flower.