GROWTH 471 



tive processes. Usually we have therefore a great aooumulation, 

 which is primarily manifested in growth. Here again we must 

 distinguish between the growth or increase of the living sub- 

 stance and the manufacture of other products such as wood, 

 which is not necessarily living. 



The growth of the living substance is always the result of 

 constructive metabolism, and is attended by an increase of bulk 

 and weight. The growth of an organ sometimes appears to be 

 independent of such increase of weight ; indeed, a diminution 

 of the weight of the whole structure is sometimes noticeable. 

 Thus, in the case of a potato allowed to germinate in such con- 

 ditions as prevent the absorption of food materials from without, 

 we have a marked change of form, but owing to the loss of 

 moisture by transpiration and of COj as a consequence of its 

 respiration, or the catabolic processes going on in it, the result- 

 ing plant may weigh much less than the original potato. 



This difference is, however, rather apparent than real. We 

 shall see that the actual growth, or the manufacture of new 

 cells, is confined to certain regions. In these regions there is a 

 considerable increase in bulk and weight, but as the materials 

 which are used for the purposes of this local growth are derived 

 from substances stored up in the body of the tuber, the latter, 

 which is not the seat of the growth at all, 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 considerable growth may have taken 

 place. 



Growth is in the strict sense, then, always associated with 

 the formation of new substance ; it is in nearly all oases 

 attended by a permanent change of form. This is perhaps not so 

 evident in the case of axial organs, though here it takes place to 

 a certain extent, as it is in that of leaves and their modifications. 

 In most cases the young leaves of a plant have a different shape 

 from the adult ones, and the appearance of the latter is gra- 

 dually assumed as the leaf grows older. 



This change of form not only can be seen in the case of 

 such an organ as a leaf, but may be noticed also in that of the 

 individual cells of which a plant consists. In the apical 

 meristem of the shoot of a flowering plant the cells when first 

 formed are almost cubical ; after a little while we find many of 

 them becoming prosenchymatous. Many other cases can be 

 noted, particularly the irregularly shaped cells of the spongy 



