CHAPTER XII. 



VEGETABLE GROWTH. 



988. As already shown, vegetable growth consists (1) In the 

 formation of new cells, (2) in the increase in size of previously' 

 existing ones, or, (3) as is commonly the case, in both of these 

 processes taking place simultaneously. In the production of 

 new cells and in the augmentation of cells in size there are cer- 

 tain chemical and physical phenomena which alwaj's accompany 

 the morphological changes. 



989. The chemical changes are essential!}' those which have 

 been described under Transmutation and Respiration ; available 

 matters change their character in order to be utilized in the for- 

 mation and increase in size of cells. The phjsical phenomena 

 are chiefly those which accompany oxidation ; namely, the evolu- 

 tion of heat and the production of electrical disturbances. 



990. The materials used by the plant for the formation of new 

 structures are produced by assimilation ; and in annuals a large 

 part of the assimilated matter is consumed in growth as soon 

 as it is made. But, in perennials, especially in those which 

 belong to climates where vegetation has periods of rest, a por- 

 tion of the assimilated matter is stored up for future use. The 

 rapiditj' of the growth from buds in the spring is due to the 

 abundant supply of assimilated matters prepared during the pre- 

 ceding summer. 



991. Hence growth is not necessarily associated with increase 

 in weight. In fact, in the growth of new parts from a bulb or 

 tuber, although there is a marked increase of volume, there is, at 

 first, an actual loss of dry substance through oxidation. More- 

 over, one part may grow at the expense of another ; and we maj' 

 have under certain conditions the anomaly of an increase in 

 volume of new organs, with simultaneous but larger decrease in 

 size of older parts, so that the result, as regards the whole, is 

 diminution of weight. 



992. Morphological changes in the cells. The two processes 

 involved in ordinary growth, namely, increase of cells in number 

 and in size, maj' go on together. But growing cells belong to 



