ELEMENTS OK STRUCTURAL BOTANY. 107 



chiefly required by the plant. Now it is a matter of 

 common experience that, with rare exceptions, a plant 

 will wither and die unless supplied with adequate mois- 

 ture. We therefore come to the couclusiou, that at 

 any rate the greater part of the nourishment of plants 

 is imbibed in liquid form through the roots. The law 

 of endosmose, in accordance with which this imbibing 

 goes on, has already been explained. Tlie sop, as it is 

 called, ascends through the newer tissues, and is at- 

 tracted to the leaves by the constant evaporation going 

 on there, and the consequent thickening of the contents 

 of the cells in those organs. 



173. And this leads to the question— How does the 

 water-vapour make its escape from the leaves ? The 

 microscope solves this difficulty for us. A leaf almost 

 always presents one surface towards the sky and the 

 other towards the ground. It is protected on both sides 

 oy an epidermis or skin, consisting of very closely 

 packed cells. The side exposed to the sun is almost 

 unbroken, but the lower side is seen, under 

 the microscope, to be perforated by innu- 

 merable little openings, which lead into the 

 body of the leaf. These openings, to which 

 the name stowala, or stoindttti (Fig. 170) 

 has been given, have the power of expanding 

 when moistened by damp air, and contract- 

 ing when dry. By this wonderful contriv- 

 Fig. 170. ance, the rate of evaporation is regulated, 

 and a proper balance maintained between the supply 

 at the root and the loss from the leaves. The stomates, 

 it may be noticed, serve also as means whereby carbonic 

 acid may be directly absorbed from the air. In those 

 plants whose leaves float on water the stomates are 



