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PHYTOGENESIS. 251 I return then to my question: what is the meaning of to grow? In hackneyed phrase we are told, “To grow signifies increase of the mass of an individual, and takes place in the inorganic world by juxtaposition, in the organic by intussusception.” Have we gained anything for vegetable physiology by this reply? I think not. If the plant is to grow by intussusception, then I say it consists of an aggregate of single, independent, organic molecules, the cells; it increases its mass by new cells being deposited upon those already existing; consequently by juxtaposition. But the single cell in the progress of its expansion, which frequently reaches an enormous bulk in comparison with its original size (I will merely remind the reader of the pollen-tubes), also increases in substance in the interior of its membrane, and by this means also the mass of the entire plant is increased; it consequently grows by intussusception also. Finally, after a certain period the cell deposits new organic material in layers upon its primitive membrane; thus another form of juxtaposition, which still, however, belongs to the cycle of vegetable vitality. It hence becomes readily apparent that, in respect to scientific botany, the idea “grow” still requires a new foundation in order to be capable of being applied with certainty.

Of the three instances just cited, the second and third belong more to the individual life of the cells, and are of secondary importance only, as respects the idea of the whole plant, regarded as an organism composed of a certain number of cells. The plant considered in its totality increases its mass, that is, the number of the cells composing it, in the first way only.

We must therefore here discriminate three processes essentially distinct from each other in a physiological sense, which, when strictly regarded, scarcely find an analogy in the other kingdoms of nature.

1. The plant grows, that is, it produces the number of cells allotted to it.

2. The plant unfolds itself by the expansion and development of the cells already formed. It is this phenomenon especially, one altogether peculiar to plants, which, because it depends upon the fact of their being composed of cells, can never occur in any, not even the most remote form in crystals or animals.