Because it gives back, with other substances, a considerable proportion of silica, in that form which adapts it to the use of the succeeding crop.
1265. Why is the structure of herbaceous plants less consolidated than that of woody plants?
Because, for the most part, herbaceous plants last only a single year; they, therefore, do not require the enduring qualities of plants that have to sustain the influences of the elements for a succession of seasons.
1266. Why are the stalks of plants of light structure generally cylindrical?
Because the cylindrical form is stronger than any other; a hollow cylinder, with moderately thick walls, is stronger than a solid rod, containing the same amount of material.
1267. Why do the stalks of plants become hollow?
Because the parallel and perpendicular fibres of the stalk are developed more rapidly than the horizontal. The growth of the plant, therefore, consists of a kind of divergence from the centre.
1268. Why are the stomata, or pores of leaves, generally placed on their under surface?
Because, being placed on the under surface, they are shaded from the action of the sun's rays, and so carry on the function of respiration more actively than if subjected to direct heat; they are also protected from the injurious effects of dust; and are moistened by evaporation from the earth's surface.