366 PHYSIOLOGY 
toward the chloroplasts, where the CO 2 is actually utilized. Here, 
indeed, is the point at which the normal pressure of CO 2 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 CO 2 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 CO., and H 2 O. The carbon dioxid and water enter into 
a double relation. In part, the CO 2 is merely dissolved in the water; 
in part the two form a loose chemical combination, carbonic acid, H 2 CO 3 . 
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, chlorenchyma, 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 Conjugates, 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 
