366 PHYSIOLOGY 



toward the chloroplasts, where the CO2 is actually utilized. Here, 

 indeed, is the point at which the normal pressure of CO2 usually limits 

 the process of photosynthesis. The main-line transportation through 

 stomata and intercellular spaces is adequate, but the switching facilities 

 in the terminal yards (from cell wall to chloroplast) are not; hence when 

 otherwise capable of operating to full capacity, the laboratories are 

 hindered by the impossibility of securing enough of this raw material. 

 There are other factors which may limit the output, to be discussed 

 later; but the shortage of CO2 due to low diffusion pressure is the com- 

 monest. 



Water. — Water, the other of the raw materials, is never lacking when 

 plants are active. Its source for most land plants is the soil water that 

 enters through the roots. The little that may enter via the leaves (com- 

 parable with the amount leaving in the same time by cuticular evapora- 

 tion, p. 327) is practically negligible. Only in mosses, liverworts, and 

 a few epiphytes, i.e. plants with practically uncutinized surfaces, may 

 it freely enter aerial parts. In many such cases there are special struc- 

 tures that hold water until it can enter. 



Relation of CO2 and HjO. — The carbon dioxid and water enter into 

 a double relation. In part, the CO2 is merely dissolved in the water; 

 in part the two form a loose chemical combination, carbonic acid, H2CO3. 

 This three-phase system, solute, solvent, compound, is in equilibrium, 

 and if the amount of any member is altered, corresponding changes take 

 place in others and equilibrium is again reached. 



(2) The Laboratories 



Chloroplasts. — The laboratories in which photosynthesis proceeds 

 are the chloroplasts. These are organs of various form and size, found 

 only in superficial parenchyma cells, chlofenchyma, of stems and foliage. 

 (For a discussion of this tissue and its relations to external agents, 

 see Part III, p. 530.) The chloroplasts are embedded in the cytoplasm 

 just within the ectoplast and marked by their green color. In a few algae 

 (especially the Conjugales, p. 37) they have various and sometimes 

 fantastic forms, but in almost all the higher plants they are shaped like 

 a bun or a thick round cake; that is, two diameters are nearly equal, 

 and the other is shorter, with the convexity greater on one face than the 

 other (see fig. 619, p. 297). Their form is subject to change from internal 

 causes, and in moving about with the cytoplasm they are easily distorted 



