1060. The sap in plants has been compared to the blood in animals; as it is by means of the sap that nourishment is supplied to the plant. All the food taken up by the roots must be in a fluid state; and this fluid, as soon as it has entered the plant, is called the sap. As it rises upwards it fills the cells, and by supplying them with an excess of nutriment, causes them to produce new cells ; and thus the plant grows, that is, increases in size, by the lengthening and widening of its cellular tissue. In the course of the passage of the sap upwards, it becomes partially decomposed, and several of the elements which it contained are deposited; forming the various kinds of tissue which are required by the plant in its progress towards maturation.