Popular Science Monthly/Volume 50/November 1896/Science in Wheat-Growing
|SCIENCE IN WHEAT-GROWING.|
THE chlorophyll cells and the leaves of the plant may be regarded as little laboratories elaborating vegetable matter; they work upon the carbonic acid, which the enormous quantity of water they contain enables them to extract from the atmosphere, reduce it, and form with the residue from its decomposition, after the elimination of oxygen, sugars and cellulose, straw-gum, vasculose, and all the ternary matters composed of carbon, oxygen, and hydrogen; these cells likewise reduce the nitrates which are brought to them at the same time with phosphoric acid, potash, and silica, by the water which constantly traverses the plant, entering it at the root and being exhaled from the leaves.
If rain is frequent and the soil well moistened, the cells will continue their work for a long time; they will elaborate much vegetable matter, and the plant will grow. But the course is not the same if rain is scarce and the soil is parsimonious in providing for the enormous expenditure of water which the wheat makes. I have found that a leaf of wheat exhales, under one hour of insolation, a weight of water equal to its own. When the earth, insufficiently watered by rain, becomes incapable of supplying this prodigious consumption, desiccation of the organs is produced, and it is always the oldest leaves which dry up and perish first. A May rarely passes without one seeing the little leaves fixed at the base of the stem soft, flabby, and withered. If we submit them to analysis, we find that they have let escape some nitrogenized matter, phosphoric acid, and potash, which they contained while they were living, green, and turgescent. It is well to lay stress upon this death of the leaves, and on the departure of the materials they contain; when the leaf dies, one of the small agglomerations of working cells is closed, the quantity of matter elaborated is then less than if it had continued its task, and as the closure of these little laboratories is determined by their desiccation, we conclude that the quantity of vegetable matter formed during dry years is limited, and that the stems are shortened and there is little straw.
At the moment when desiccation begins the nitrogenized matter which forms the protoplasm, the living part of the cell, is metamorphosed, and takes an itinerant property that permits it to pass through the membranes and migrate toward the new leaves, carrying with it its usual accompaniment of phosphoric acid and potash. This transportation of some of the elaborated material from the lower leaves toward the upper leaves goes on through the whole duration of vegetation, and continues at the time of flowering, which, without doubt, by a mechanism of which we do not know the method of operation, takes place only when the quantity of materials elaborated is sufficient to nourish the seeds which are about to appear.
The wheat begins to head, in our latitude, early in June. On pressing lightly between the fingers the upper part of the stem, at the place where it appears a little swollen, we meet a slight resistance, due to the head, which is entirely formed before it emerges. It is composed of a stem; the rhachis, which bears the flowers, formed of little green leaflets; and the glumes, one of which terminates, in some varieties, in the long appendage characteristic of bearded wheat. If, at the moment when the head emerges outside of the stem, we gently lay open the glumes, we shall discover the essential organs of the flower within. On a little greenish swelling, the rudiment of the corn, are fixed two little aigrettes of plumes, slightly divergent. These are the pistils, the female organs. Around them, fixed at the extremity of fine peduncles, are the anthers, as yet closed. They contain the pollen, the yellow fecundating dust. At the moment of maturity the anthers open and the pollen falls on the little plumes of pistils, well constructed to hold it. It germinates there, sends out a long tube—the pollinical branch—into the ovule, to which the plumous pistils are attached. Fecundation is accomplished, and the corn is formed.
All these delicate operations, which it is so interesting to follow, take place in the formed flower. When the stamens, emerging between the glumelles, appear without, or, to use the common expression, the wheat is in flower, everything is really done. So, when we try to create hybrids—that is, new varieties—endowed with qualities wanting in one of the parents, we must take the anthers from the flowers before the plumes are open and the anthers have shed their pollen.
The operation exacts much care. When the flower is half opened, we cut off the anthers it contains and drop in the pollen of the variety which we have chosen to give the one we operated upon the qualities which it lacks. One of the most widely distributed varieties around Paris, the Dattel, was created in this way by M. H. de Vilmerin by fecundating the pistils of the English Chiddam wheat, which had fine qualities but a short straw, with the pollen of the Prince Albert wheat. The operation was perfectly successful; the straw of the Dattel is thicker and longer by at least five inches than that of the Chiddam, from which it is derived. The variety is quite fixed; it reproduces itself with well-defined characteristics; and the experiment has now been of long enough duration to make it certain that the seed sown is not derived from plants reverting to the parental characteristics, as sometimes happens in imperfectly fixed hybrids.
When flowering takes place in good weather, fecundation goes on regularly, and the chances increase of obtaining a good crop. These chances, on the contrary, diminish when the earing occurs in a rainy time. Probably water gets within the involucre, and the wet pistils imperfectly retain the pollen grains, or their germination is irregular, the pollinic branch not reaching the micropyle, and the ovules not being fecundated; and the ears bear many sterile flowers in which the corn is not formed.
The production of the corn, of the seed which assures the perpetuity of the species, is the ultimate end of the herbaceous plant; it is essential that the reserve stores necessary for its development be accumulated around the embryo inclosed in this seed, and that it find everything near by: the starch which it will liquefy and then transform into cellulose; the gluten, the nitrogenized matter, with which it will form the protoplasm of its cells. These reserves must be abundant, so that a part of them may be burned, producing by their slow combustion the heat which favors these transformations. The whole life of the herbaceous plant tends toward this end of accumulating in the seeds the principles elaborated during its short existence; and it is precisely this accumulation in the seed of the gluten and the starch, both excellent food-stuffs, for which men have cultivated wheat from the most remote antiquity; or, if they live in different climates from ours, sow other corn plants—rice in the extreme East, maize in America—in order to find in their seeds the association of nitrogenized matter and starch which gives the grain so pronounced an alimentary value that it forms an essential part of the food of a large proportion of the inhabitants of the globe.
It is easy to follow the migration of the nitrogenized matter, phosphorus, and potash from the lower to the upper leaves, and from these to the end of the stem and the seed. The transportation of these principles has been studied for more than thirty years by a distinguished agronomist, Isidore Pierre, professor in the Faculty of Sciences at Caen. We are less well informed concerning the formation of starch. It can not be seen accumulating in the leaves of wheat as in those of a large number of other species, nor are reserves of saccharine matters found in these leaves. The formation of starch is very late, as it does not take place till during the last stage of vegetation. It thus happens that the quantities of starch contained in the grain vary greatly from one year to another.
The phenomenon of transportation and migration of nitrogenous substances from the leaves and the stem to the grain and the later production of starch takes place only when the plant conserves a considerable quantity of water. If the radiations of a burning sun strike upon a field of wheat the roots of which find nothing to drink in a dry soil, the plant dries up, everything stops, and the last phase of the life of the wheat is abruptly terminated; the grains remain empty, and the crop fails.
Persistent rain is no less to be feared. The wheat continues to grow indefinitely, and the migration of the principles is not brought about. I witnessed a very curious example of this in England twenty years ago. I was visiting a farm near London, where cultivation was assisted by irrigation with sewer water. The farm was slightly undulating, and the sewer water was carried over the depressions in troughs sustained a few yards above by wooden supports. One of these troughs, being in bad condition, let the liquid fall constantly in a fine rain upon several square yards of a field of wheat. It was July, and, while all the rest of the field was yellow and ready for the harvest, the stools thus watered were still green and continuing to grow, exceeding all their neighbors in height, and giving no signs of maturity.
A mild temperature and a slightly clouded sky are the favorable conditions for a good ripening. When the land has been well dug, the seeding regular, and the manure judiciously distributed, all the individual plants in the field will have expanded together, all will have passed simultaneously through all the phases of their development, and in the warm hours of the day, when all is motionless, the surface of the field, the English say, will appear as horizontal as a table.
There are no great inconveniences in harvesting a little early. The ripening, if not yet complete, will proceed very well when the sheaves are stood up against one another into those "shocks" which are much in use where severe rains are common. On the other hand, there is much advantage in not leaving the wheat standing after it has ripened. Every plant that has matured its seed tends to shed it, and sometimes the seed has powerful organs of dissemination. This is not the case with wheat; but, although it does not fly off to a distance, it escapes from overripe heads, falls, and is lost. Further, all the organs of plants respire by the aid of the oxygen of the air consuming some of their principles. In the seed the combustion chiefly affects the starch, and a crop which remains standing long diminishes in weight both by the loss of the seeds that fall and by the slow combustion which continues as long as desiccation is not produced. As soon as a field of wheat is ripe it should, therefore, be harvested, and here is where the reapers, that have been brought to such great perfection in America and England, are found to be very useful in making the farmer independent of the scarcity or the exactions of laborers.—Translated for the Popular Science Monthly from the Revue des Deux Mondes.