420 MANUAL OF BOTANY 



the axial portions of the plant. Where there is a large flow of 

 water, as in a tree, there is a continuous formation of material ; 

 where the transpiration is but slight, as in a Cactus, there is but 

 little formation of new plant substance. 



CHAPTER III. 



THE SKELETON OF THE PLANT. 



We have seen that the protoplasm of the plant is a soft almost 

 jelly-like substance, incapable of resisting any considerable 

 disturbing force or pressure, such as almost all plants are 

 inevitabl.y exposed to, whatever be the nature of their environ- 

 ment. To enable them to cope with the difhculties presented 

 by the latter, plants must possess a certain rigidity, which varies 

 with the nature of their situation. The soft protoplasm must 

 consequently be supported by something which will secure this 

 rigidity. The several protoplasts are in most cases surrounded 

 by a cell-wall or membrane which subserves this purpose. 

 According to the habitat of each plant the development of its 

 cell-membranes shows various degrees of complexity, and many 

 modifications of the original structure are found. We have 

 already seen that such modifications take the forms of alteration of 

 the shape of the cell, of aggregations of cells in various ways, and 

 of differences of chemical constitution brought about as the age of 

 the cell increases. These differences are not merely connected 

 with the maintenance of rigidit3^ but subserve also various 

 functions which the different tissues of complex plants perform. 

 In the simplest plants, consisting of single cells, or rows or 

 plates of cells, and living in water, but little rigidity is needed. 

 The unthickened cell-wall is here sufficient, when combined 

 with the turgescence of the cells, to prevent their being destroyed 

 by the ordinary antagonistic influences wdiicli they have to 

 encounter. The form of the j)lant is also modified to enable it 

 to survive such difficulties. Plants growing in rapidly flowing 

 water usually have flexible stems and much divided leaves, 

 which consequently give way to the current and escape damage. 

 Those of terrestrial habit, on the other hand, exposed to winds 

 and storms, are fmniished with considerable development of 

 woody roots which anchor them securely to the earth, and with 

 sturdy trunks consisting of masses of wood, which secure 

 stability even when atmospheric disturbances are severe. The 



