306 



VEGETABLE PHYSIOLOGY 



going on in its tissues, the resulting plant weighs much 

 less than the original potato. 



This difference is however rather apparent than real. 

 We shall see that the actual growth, as well as the manufac- 

 ture of new cells, is confined to certain regions. In these 



regions there is a conside- 

 rable 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- 

 siderable growth may have 

 taken place. 



Mere increase of weight 

 in an organ does not, on 

 the other hand, necessarily 

 imply any growth.. The 

 deposition of reserve mate- 



FIG. 135. LONGITUDINAL SECTION OF 



YOUNG ROOT, SHOWING STRUCTURE OF rials in many Seeds does 

 GROWING POINT, x 20. , n -, 



not take place to any great 



1, zone of cell division ; 2, region of greatest , . 



growth ; 3, region of complete differen- extent till their mature 

 tiation. 



dimensions are reached, 



and growth is therefore completed. It involves, however, 

 a considerable increase of weight. 



Growth is in the strict sense, then, always associated 

 with the formation of new living substance, and 



