CHAPTER X 

 GROWTH . 



115. Definition. — Growth is increase in size of either the 

 organism as a whole or of any of its parts. By growth the 

 individual protoplast of a cell may become more bulky, 

 the chloroplasts or leucoplasts may become larger, the 

 nucleus bigger, the cell-walls thicker, the cell as a whole 

 may increase in any dimension, and, as a result of this, 

 the tissues and organs, and finally the entire organism, 

 may become larger. Growth does not always involve the 

 whole organism. Cell-walls often grow thicker while the 

 size of the plant as a whole does not alter. Growth often 

 involves a decrease in the size of one or more of the parts; 

 thus, when a potato "sprouts," the tuber itself, giving its 

 substance to nourish the newly formed stems, becomes 

 smaller and Hghter in weight, while the stems increase in 

 size and weight. As a whole, however, the potato plant 

 is growing (Fig. 60). 



116. Osmotic Pressure and Growth.— Growth does 

 not always involve increase in weight. If, for example, 

 the osmotic pressure increases within a turgid cell, and if 

 the cell- walls are elastic, the cell will grow bigger in one or 

 more dimensions. Not only may this growth involve no 

 increase in weight (as, for example, when the increase in 

 osmotic pressure within a cell is due merely to the chang- 

 ing of starch to sugar), but may even be accompanied by 

 loss of weight on account of waste products being given 



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