Silicon is always considered as a non-essential element, although it occurs in such large proportions in some plants as to indicate that it cannot be wholly useless. It accumulates in the stems of plants, chiefly in the cell-wall, and has sometimes been supposed to aid in giving stiffness to the stems. But large numbers of analyses have failed to show any direct correlation between the stiffness of straw of cereal plants and the percentage of silicon which they contain. Further, plants will grow to full maturity and with erect stems when no silicon is present in the mineral nutrients which are furnished to them. On the other hand, certain experiments appear to indicate that silicon can perform some of the functions of phosphorus, if soluble silicates are supplied to phosphorus-starved plants. But under normal conditions of plant nutrition, it seems to have no such function.
There are a variety of plants which furnish products known as hemp, but commercially only three are of much importance, the plant universally known under that name, the Manila hemp, and sisal. All of them are used chiefly for cordage. The hemp of the ancients is a tall annual related to our nettles, with rough leaves, and a native of Asia. For centuries an intoxicating drink was made from the herbage of this plant, and this with the narcotic hashish, which is made from a resin exuded by the stems, obscured the fact that Cannabis sativa is a very valuable cordage plant.
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Upon the ability of like to produce like rests the continuity of those plant groups, well exemplified by Lycopodium Selago and the ginkgo, which, through all the changing panorama of the history of the plant world, have steadily produced individuals so close to the ancestral type as to be essentially indistinguishable from it. It is upon the possession of this ability that all the different races of plants depend for the unchanged perpetuation of their kind. And, as we shall presently see, it is also upon this very ability that the new forms that do arise, rely for holding fast to their differences.