STRUCTURE OF MESOPHYLL 115 



the chemical laboratory. For the first process consider- 

 able energy is required, and it is this that certainly 

 requires, in the chloroplast, the radiant energy of 

 light. This first process may then be designated as 

 photolysis, the second process, that of condensation to 

 form sugar being a process of chemo synthesis. For 

 ordinary biological purposes, however, we may consider 

 them together as photosynthesis. 



Structure and Functions of the Green Plant Cell. 

 The green tissue (mesophyll l ) which forms the bulk of 

 an ordinary foliage leaf (see Fig. 10) consists of cells 

 with thin cellulose walls lined inside by a layer of 

 cytoplasm enclosing the nucleus and numerous chloro- 

 plasts, and with a large central vacuole. These cells 

 are, in fact, " adult " living cells (see p. 101) containing 

 chloroplasts (Fig. n). 



The mesophyll tissue is covered by a layer of colour- 

 less living cells (epidermis) on the surface of the leaf, 

 and the outer walls of these cells, in contact with the 

 external air, has a waterproof layer of cell wall sub- 

 stance (cuticle) which prevents the cells of the leaf 

 drying up by evaporation to the air (Fig. 10, st.) The 

 epidermis (and cuticle) is, however, pierced by a number 

 of holes (stomata 2 ) which lead from the external air 

 to the air-containing intercellular spaces between the 

 mesophyll cells. These intercellular spaces are inter- 

 communicating, and are so arranged that every meso- 

 phyll cell has some part of the outer surface of its wall 

 in contact with the internal atmosphere of the leaf. 



The internal atmosphere of the leaf, being in com- 

 munication with the external air through the stomata, 

 has at first the same composition as the latter, 

 but this composition is continually being modified by 



1 Greek jue'aoj, middle, and ^vAAov, a leaf. * oro/ia, mouth. 



