320 



PLANT PHYSIOLOGY 



After elongation has occurred and the cells have reached thek 

 mature size, differentiation begins. Up to this time, all the cells 

 look very much alike. Now the cells which are to be tracheids 

 become thicker walled and assume their characteristic markings; 

 the cells which are to become sieve tubes develop pits in the end 

 walls by the resorption of certain parts of the wall, leaving holes; 

 and cells which are to form the strengthening tissue take on the 

 characteristics of the mature cells. Increase in size is now at an 

 end or takes place very slowly, ultimately stopping completely. It 

 is this phase of growth, — the maturation (differentiation) period, — 

 which is considered by some writers as the most important phase 

 and by others hardly to be a phase of " growth " at all. 



Grand Period of Growth.— During the period of formation 

 there is very little enlargement, and the same can be said of the 

 period of maturation. The curve of growth is thus an S-shaped 



cm 



time 



Fig. 20. — The typical S-shaped curve of growth. Growth starts in slowly, 

 then speeds up during the elongation period after which it slows again during 

 maturation. 



curve as seen in Fig. 20. Growth starts in slowly during the form- 

 ative period, then speeds up enormously during the elongative 

 period, and finally slows down again during maturation. This 

 central period when growth (increase in size) is the most rapid 

 is called the grand -period of growth, and is found not only in the 



