GROWTH 203 



Having now grasped in general outline the influence 

 of the chief external agencies, light, heat, and the 

 attraction of the earth, let us try and study more closely 

 the very essence of the phenomena in question. So 

 far we have investigated the plant as a whole ; but the 

 life of the plant is the sum total of the life of innumer- 

 able cells. Let us see how the development of cells 

 is related to the general phenomena of growth. We 

 know that every cell in the course of its life-time increases 

 in size, changes its shape and the structure of its walls 

 — in a word, grows. We also know that whatever be 

 the size of the plant, it commences as a single cell and 

 eventually contains millions of them. It is evident 

 that the growth of a whole plant depends upon 

 two phenomena, the growth of single cells and their 

 reproduction. 



It is now necessary for us to catch a glimpse of how 

 these phenomena of the growth and reproduction of 

 cells, which underHe the growth of the plant as a whole, 

 take place. We must choose for the purpose appropriate 

 material, an organ or organism in which we can study a 

 living cell without disturbing it. We find such appro- 

 priate material in the filamentous weeds that form the 

 main part of what is generally known as ' green slime.' 

 If we place one of these green filaments under a 

 microscope, we shall see that it consists of a single 

 straight row of cells. Fig. 58 shows at the top such a 

 cell with the green matter we have called chlorophyll, 

 which gives plants their green colour distributed in a 

 very characteristic way. Here it forms green bands 

 with toothed edges, twisted spirally round the inner 

 surface of the cell-wall : hence its Latin name of 

 Spirogyra. Apart from this peculiarity the cells of 

 Spirogyra do not differ in any way from the ordinary 

 type of a cell. The same wall of cellulose is found in 



