GROWTH. 43 



considered to be the result of the greater amount of work 

 done (under the influence of sunlight) through the medium 

 of the green. cells than of that accomplished by the colour- 

 less cells, even though their action is continuous, and that 

 of the green cells intermittent. It is for us now to inquire 

 how this increase of substance, how this growth and build- 

 ing up of new materials takes place. The circumstances 

 that are propitious, or which are antagonistic to it, have 

 been considered; it remains to enquire into the way in 

 which the process itself is effected, and, for this purpose, 

 we must revert to the fabric of the plant, and go back to 

 the cell. 



Growth of Cells. The perfect cell, as has been 

 explained, consists of a membranous bag enclosing the 

 protoplasm, some liquid contents, and in the growing state 

 a small highly refracting oval body known as the 

 "nucleus." The growth of a cell may take place in one 

 of three different ways. There may be simple extension of 

 the cell membrane, which becomes stretched by the influx 

 of fluid into its cavity, producing a state of " turgescence " 

 in the cell. But this turgescence, which is of intermittent 

 occurence, is of itself hardly to be truly considered as growth 

 in the sense of the formation of new material, although so 

 closely associated with it that no true growth can take place 

 without it. 



A second mode of growth, resulting in the real addition 

 of material, and consequent increase of weight, is that called 

 " intercalary," because new material is supposed to be inter- 

 calated or squeezed in between the old. Thus, the molecules 

 of cell-membrane are separated by the pressure caused by 

 the turgid protoplasm, and into the interstices so formed, 



