168 



PLANT PHYSIOLOGY 



sap, which accumulates in large vacuoles, without there being 

 any considerable increase in the amount of protoplasm in 

 each cell. Figure 10 corresponds to region 3 of figure 7, and 

 shows how the cells change in taking on their definitive 

 characters, Ves. indicating some of the changes taking place 

 during the formation of a vessel in the central vascular 

 bundle. In this region the amount of protoplasm decreases 



Ep. Cor. Ves. 



FIG. 7. FIG. 9. FIG. 10. 



Figures 710. Longitudinal sections of Azolla root. Fig. 7 diagram- 

 atic showing region of cell-formation (1). cell-growth (2). cell-differenti- 

 ation (3). Fig. 9. = 2 in Fig. 7 more highly magnified. Fig. 10. = 3 in 

 Fig. 7 more highly magnified. 



not only proportionally but absolutely. There can be little 

 more if any permanent increase in volume in this region, 

 although there may be increase as well as decrease in vol- 

 ume because of differences in turgor solely. 



The factors contributing to make growth possible may be 

 grouped under three heads : 1st, there must be an adequate 

 supply of material ; 2d, there must be an adequate amount 

 of room ; 3d, there must be the impulse. Physiologists are 

 not able to reduce to definite physical and chemical terms 

 what is comprehended under this last head, and it has still 



