Page:Popular Science Monthly Volume 50.djvu/116

104 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