46	THE FUNCTION OF VEGETATION
contact with the wet surfaces of the soil particles. The cellular membranes which clothe the root hairs and rootlets being from the nature of the case perfectly permeable to water, it follows that, if the root be exposed to air which is not saturated, it will give off moisture and dry up. The soil roots of a plant which has become partially or completely uprooted thus constitute its most vulnerable spot. In practice only roots which have developed in situ are of any service, and for their continued efficiency it is evident that they must not be disturbed.
   A very important consequence of the intimate relations that obtain between all the ramifications of a root and the soil in which it grows is the firm fixing of the plant in the ground, so that the plant is hardly liable to be uprooted Fig. 8.—Diagram of a Plant in the Soil even by the most tempestuous
The root (r) with it. absorbent root hair, (h) i«	buffeting by wind, expanded in the soil, the interstices of which are The shoot Oil the Other
indicated. The stem (s) and leaves (l) are raised	,	.
aloft. The heavy black edge represents the cuti-	hand) IS mainly developed
cularized impermeable surface membrane of the aKove ^rOUnd where it dis-shoot. The gaps are the stomata, the points of dUUVC gTUUIlU, Wlierc 11 Ulb
gaseous interchange. The root system, bordered plays tO the light itS greet! by a thin line, is water-permeable. G.L., Ground	r	T
icvei.	canopy of living cells. It is
                                  by the agency of its leaves and other green parts that the plant is able to utilize the C02 of the air as its source of carbon; moreover, it is also through the leaves that water vapour is transpired into the air. These functions are carried out under cover of a non-permeable membrane (the cuticle) by special adjustable pores, the stomata, which lead into the interstices of the plant (fig. 8). As these pores can be closed and communications interrupted when necessary there is little fear of a plant drying up.
   We see, therefore, that both the root and shoot of a plant are necessarily permeable to water, but that, whilst the root