1056. The cellular tissue is, perhaps, the most important of all the elementary organs of plants, because it is the principal agent in their additional developement. In fact, what is called growth, is a rapid increase of the cells, as each cell contains within itself the power of forming a new cell when the nutrient matter contained in the first increases beyond a certain quantity. 'In every actively vegetating part of a plant,' says Schleiden, 'exists a continuous influx of new nutrient matter which is absorbed by the roots, while its superfluous water is evaporated through the stomates.' This movement of the sap is very rapid; and as each cell becomes surcharged with a quantity supplied to it, secondary cells are formed within the first, and then 'the mother cell dissolves and disappears, while the two, four, eight, or more, young cells, produced by it, occupy its place. The whole process, which we call growth, in plants, consists, in its essential elements, of a continuous propagation of cells of this kind ; whence the number of cells becomes multiplied beyond calculation, nay, almost beyond credibility. From an approximative calculation, for example, in a rapidly growing fungus, the Bovista gigantea, 20,000 new cells are formed every minute!' The cellular tissue of plants has been compared to the flesh of animals.