GROWTH." 45 



chlorophyll in its tissues (and it is with these alone that 

 we are here concerned) gains carbon in the form of car- 

 bonic acid gas when it is exposed to light, and loses it 

 constantly, whether in light or darkness. Nevertheless, 

 as the total gain is greater than the loss, the balance is in 

 favor of the plant. The plant may thus be 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 color- 

 less cells, even though their action is continuous, and 

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

 enquire how this increase of substance, how this growth 

 and building up of new materials takes place. The cir- 

 cumstances that are propitious, or which are antagonis- 

 tic 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 "tur- 

 gescence " in the cell. But this turgescence, which is of 

 intermittent occurrence, 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 intercalated or squeezed in between the old. Thus, 



