66 



GENERAL BOTANY 



7. 



GROWTH OF OTHER PLANT ORGANS 



The same phenomena which we have outlined in the growth 

 of the root obtain in the growth of all other plant organs, 

 including leaves, buds, and stems. 



Cell division and cell enlargement are the two main phenomena 

 of all growth which are demonstrable by scientific methods. In 

 the case of plants the enlargement of cells in growth seems to 



be mainly due to the absorption 

 of water and the inflation of the 

 cells by osmotic pressure. By this 

 means very rapid growth can take 

 place without the expenditure of 

 the energy necessary to manufac- 

 ture great quantities of living sub- 

 stance with which to fill the cell 

 cavities of the expanding cells. In 

 the slower-growing animals the 

 water-inflation method of cell 

 growth does not exist to any such 

 extent as it does in plants, since 

 the latter need to adapt them- 

 selves quickly to the changing 

 seasons, which necessitates the 

 rapid production of extensive root 

 and leaf surfaces for the absorption of water and gases at the 

 opening of each warm season. The growth of leaves in spring 

 is a good illustration of this aspect of growth in plant organs. 

 A large tree will produce in a few days in spring many 

 square yards of leaf surface ; this involves the production and 

 inflation of thousands of new cells. It is obvious that the tree 

 would be wholly unable in this short time to manufacture 

 enough new protoplasm to fill the tremendous space caused 

 by the expansion of the cell cavities of its leaf cells. The 

 water-inflation method is therefore both economical and neces- 

 sary in all rapidly growing plant organs, such as leaves, stems, 

 and roots. 



FIG. 35. Growth of leaves of 

 the lilac 



The spread of the squares indicates 



a uniform growth of the leaf over its 



entire surface 



