THE STEM OR SHOOT. 



139 



the buds occur in the exact position of ovules, replac- 

 ing the normal ovules, there can surely be no question 

 at all of the buds representing metamorphosed ovules 

 for the simple reason that no transitional forms or 

 stages between these buds and ovules, or between the 

 bud-leaves and virescent integuments, have ever yet 

 been met with, and it is these alone which could prove 

 the homology between the two. The buds are obviously 

 entirely new structures and bear no more homogenic 

 relationship to the ovule than do adventitious buds 

 to the foliage-leaves which bear them. 



Some have supposed that the adventitious shoot 

 arising on the inner surface of the inner integument 

 represents the nucellus ; but apart from the fact that 

 cases, as above stated, are known in which both occur 

 on the same integument, Celakovsky always found, in 

 S: Alliaria, that, while the nucellus occurred fairly 

 high up on the upper surface, the shoot always 

 occurred at the very base. 



A similarly new formation, in the above sense, is 

 the adventitious shoot produced, as described by 

 Goebel, in the place of a sporangium on the upper 

 surface of the sporophyll of the quillwort (Isoetes 

 lacustris). (For formation of flower-buds adventi- 

 tiously see under " Flowers.") 



Adventitious plants, in the form of embryos, arise 

 normally from the nucellus in Euonymus, Citrus, 

 Funlcia, and Cadobogyne. 



5. ADVENTITIOUS GROWTHS IN THE SEXUAL 

 GENERATION. 



Peothallus- Shoots. 



The sexual generation or gametophyte is, as every 

 botanist knows, differentiated as a distinct, indepen- 

 dently-growing plant among the Vascular Crypto- 

 gams, and is known as the proth alius. A new vascular 

 plant or sporophyte arises from this as a result of 

 fertilization taking place in the female sexual organ 



