28; 



support of, and channels of supply to, the cells, which are 

 the active, vital factors. It is in this way that such cell 

 systems become exposed to the light and air, which are the 

 first essentials of their activity. The air constitutes the 

 source of supply of the essential carbonic acid, while 

 the light of the sun, direct or diffused, or provided from 

 material stored up thereby in the past, forms as it were 

 the propulsive agent to the activity aforesaid through its 

 action on the chlorophyll contained in the formative cells. 

 Normally each leaf commences with the formation of 

 what is termed a mother cell, which is endowed with 

 the faculty of producing other cells, arranged on the 

 definite plan peculiar to the plants concerned, and when 

 this plan is fully completed there is engendered a series 

 of what we may term marginal cells, which close the 

 circuit, and the leaf or other structure is thus perfected. 

 A great amount of variation within specific limits is due 

 to changes in this limitation. Thus leaves normally 

 smooth-edged and of simple outline may cease to adhere 

 to this form, since the vein systems which normally 

 should close the circuits, as it were, do not do so, but 

 continue to grow on beyond the normal bounds, and 

 carrying the cell tissue with them, transform the simple 

 outline into a serrate one, and even by an extension 

 of this process produce pinnate, once divided leaves, or 

 even very decomposite or much-divided ones. This 

 process we see in many phanerogamous plants, and also 

 in ferns. In the latter alone, however, we find that this 

 capacity of extra foliar extension finds expression not 

 merely at the margins, but also at the terminal points. 

 This phenomenon may present itself to very varied degrees 

 from simple forking, to which length even phanerogamous 

 plants sometimes go, to extremely complex cases, where 

 very decomposite fronds, divided over and over again into 

 very fine divisions, have every terminal multiplied so as to 



