GKOWTH 



311 



moisture by transpiration, and of carbon dioxide as a 

 consequence of its respiration or the katabolic processes 

 going on in its tissues, the resulting plant weighs much 

 less than the original potato. 

 This difference is how- 

 ever rather apparent than 

 real. We shall see that 

 the actual growth, as well 

 as the manufacture of 

 new cells, is confined to 

 certain regions. In these 

 regions there is a consider- 

 able increase in bulk and 

 weight, but as the materials 

 which are used for the pur- 

 poses of this local growth 

 are derived from substances 

 stored up in the body of 

 the tuber, the latter, the 

 greater part of which is 

 not at any time the seat 

 of the growth, diminishes 

 in weight and size to such 

 an extent as more than to 

 counterbalance the gain in 

 the growing regions. Hence 

 the whole plant weighs less 



than the tuber, though COn- FIG. 133. LONGITUDINAL SECTION OF 

 . , , , . -, -, YOUNG ROOT, SHOWING STRUCTURE OF 



siderable growth may have GROWING POINT, x 20. 



1, zone of cell division; 2, region of 

 greatest growth ; 3, region of complete 

 differentiation. 



taken place. 



Mere increase of weight 

 in an organ does not, on 



the other hand, necessarily imply any growth. The depo- 

 sition of reserve materials in many seeds does not take place 

 to any great extent till their mature dimensions are reached, 

 and growth is therefore completed. It involves, however, 

 a considerable increase of weight. 



