GROWTH 



169 



to be ascertained whether the necessary impulse comes from 

 within or from without, whether it is inherited or is new. 

 But without the impulse there will b^no growth. 



Growth will not be possible without the needed materials 

 and space. The substances essential for growth are those 

 essential for life, but they may be grouped into two cate- 

 gories — nutritious substances, and otherwise useful sub- 

 stances. The nutritious substances furnish the materials of 

 which the protoplasmic structure and the cell-wall are built, 

 and those compounds which in respiration yield the energy 

 needed by the part to complete the first stage of growth. 



If the supply of food is constantly sufiicient during the 

 period of growth, both construction and enlargement 



C.C 



Cap. 



Figure 8. Tip of root of AzoUa showing apical cell and region of cell- 

 formation =1 in Fig. 7 more highly magnified. 



wiU be uniform, other things being equal; but if the sup- 

 ply varies in amount, the rate of growth will vary cor- 

 respondingly. It is found, for example, that the growth of 

 green and independent plants is periodic in a much more 

 marked degree than that of plants which obtain their food 

 ready-made. During the day, while food is being made 

 and accumulated in the organs photosynthetically active, 



