ADVENTITIOUS DUDS. 



479 



Ndsiurtium, and Alheninis, and is very probable for the remaining cases, that they do 

 not arise endogenously at all, but exogenously ; in fact Hansen showed that in 

 the cases mentioned cell-divisions and other changes appear in various directions 

 at the points in question, in the epidermis and cordcal tissue of leaves which are 

 already fully developed, thus transforming the permanent tissue into embryonic 

 tissue, which then forthwith forms a conical growing-point whence leaves and, 

 subsequently, roots proceed. 



If the definition above given is adhered to, to regard as adventitious all 

 growing-points which proceed from already differentiated permanent tissue, we 

 must also regard the origin of the leafy stems of a true IMoss as altogether adven- 



titious, since, as has already been said in the introductory lectures on Organography 

 and as is to be seen from Fig. 3 1 7, these arise from certain cells of the protonema 

 which have already acquired the character of permanent cells. Here, as in other 

 cases, it simply results that sharp limitations with regard to Nature are always 

 ineffectual; since, though we see in adventitious structures in the higher plants 

 something superfluous for their architecture, this no longer holds good in the case 

 of the Moss. 



In a certain sense the formation of so-called gcmmx in the simply organised 

 Liverworts, Algae, and some Fungi depends upon adventitious budding; the pro- 

 toplasm of the already fully developed cells of leaves or other organs may become 



