462 



BOTANY 



which are of the greatest importance in rendering other substances 

 available for the construction of protoplasm. 



Sources of Food 



Terrestrial green plants derive their food 

 in part from the air, in the form of C0 2 , 

 and partly from the earth, from which they 

 absorb water, holding in solution the other 

 substances which the plant needs for its 

 growth. 



In all the higher plants, except a few 

 submersed aquatics, the entrance of C0 2 

 into the plant takes place ordinarily through 

 the stomata. A typical stoma consists of 

 the two guard-cells (Fig. 450) which, un- 

 like the .other epidermal cells, contain 

 numerous chromatophores. The wall of 

 the guard-cells is much thickened above 

 and below, so that the guard-cells might be 

 compared to two short rubber tubes, closed 

 at the ends, and with the wall thicker on 

 one side, the thickened wall being in con- 

 tact. When the guard-cells are strongly 

 turgescent, the thinner part of the wall 

 naturally stretches more strongly than the 

 thicker inner face, which is forced to assume 

 a more or less concave position, leaving the 

 open pore between the guard-cells, com- 

 municating with the intercellular spaces. 

 It is probable that the presence of chloro- 

 phyll in the guard-cells is associated with 

 the production of osmotically active sub- 

 stances in these cells, the result of photo- 

 synthesis, as the stomata open normally 

 only in the light. 



It has been recently demonstrated by 

 Blackman, Escombe, Brown, and Stahl, that it is only through the 

 stomata that the gases enter the plant. It has also been shown that 

 the absorption of C0 2 increases rapidly as the size of 1 lie openings 

 decreases, and that the amount taken up by an absorbing surface 

 completely exposed is no greater than when the same surface is 

 covered by a perforated plate. Thus it has been demonstrated that 

 the diffusion of atmospheric C0 2 through an aperture 1 mm. in diam- 

 eter is 40 times greater than the rate of absorption of a free alkaline 



FIQ. 449. Two water-cul- 

 tures of Buckwheat. // 

 has no potassium. (After 

 NOBBE.) 



