828 



A TEXTBOOK OF THEORETICAL BOTANY 



Linaria vulgaris (Toadflax) is a peculiar case. Not only are the root buds 

 exogenous, but they appear in the seedling stage, and their growth replaces 

 the main shoot, which aborts early. Some buds are also formed on the 

 hypocotyl. 



This leads us to the most peculiar case of all, that of the family Podo- 

 stemaceae, whose members grow only in tropical waterfalls and exhibit the 



Fig. 828. — Dicraea stylosa. Showing the thalloid de\elopment 

 of an adventitious root bearing leaves and buds (From 

 Warjnhig, in " Die Naturliche Pflanzenfamilien.") 



most surprising variety of forms, most of which are thalloid. In their 

 vegetative parts they are utterly unlike Flowering Plants (Fig. 828). 



Their developmental histories are very various, but follow the general 

 principle that the main shoot of the seedling is abortive and is replaced by 

 the outgrowth of adventitious roots from the hypocotyl. These develop into 

 the " thallus " and give rise to branch roots, to leafy shoots and to flowers. 

 These roots are flattened and contain chlorophyll. They cling very closely 

 to the submerged rock surfaces to which they are attached by root hairs and 

 by short, exogenous outgrowths, called haptera, which resemble the hold- 

 fasts of the Ivy and serve the same purpose. In extreme cases the outgrowth 



