EXTERNAL CAUSES OF GROWTH AND FORMATION. I 309 



form has appeared, the plant reverts to the juvenile condition once more. The 

 following may be cited as examples of this phenomenon. The alga Batracho- 

 spermum has a juvenile form named Chantransia (GoEBEL, 1889) ; protonemata 

 of mosses (KLEBS, 1893), and the elongated and circular leaves of Campanula 

 rotundifolia are examples of the same phenomenon (GoEBEL, 1896, Fig. 91). 



Fig. QI. Campanula roiundtfolia. When feebly illuminated the flower-buds k are arrested ; a lateral shoot 

 A develops circular leaves. From GOEBEL'S Organographie. 



Certain Cactaceae (Opuntia, Phyllocactus) may be added to this list, since their 

 shoots flatten only in light and revert to the original radial stem-like symmetry 

 in the dark (GOEBEL, 1895 ; VOCHTING, 1894). 



Since the several organs of plants often bear the most varied relations to 



