160 HETEROPHYLLY [CH. 



vestigated by Burns 1 , gave results pointing to the same general 

 conclusion as those observations already quoted. The primitive 

 type of leaf in this plant is always a " water" leaf, but this form 

 of leaf was also produced in the autumn by all the plants, 

 regardless of any external conditions which the experimenter 

 could control. On the other hand, at the time of flowering and 

 in the summer generally, almost every plant, whether growing 

 in water or air, produced the "land" type of leaf the transi- 

 tion from the "water" to the "land" type taking place earlier 

 on strongly growing than on weak stems. The author considers 

 it evident that the aquatic environment is not the cause of the 

 division of the leaf, nor does it depend on light, temperature, 

 gaseous content of the water or contact stimulus. The only con- 

 clusion, which he considers justified by his experiments, is that 

 Proserpinaca palustris has two forms adult and juvenile; under 

 good vegetative conditions, it tends to produce the adult form 

 with the undivided leaf, the flower and the fruit, while, if the 

 vegetative conditions are unfavourably influenced, a reversion 

 can be induced to the primitive form with the submerged type 

 of leaf. These results are consistent with those of McCallum 2 , 

 who had dealt with the same species at an earlier date, but his 

 interpretation is slightly different. He is inclined to regard the 

 occurrence of the aquatic form as induced by the checking of 

 transpiration, and by the increased amount of water which hence 

 accumulates in the protoplasm. This explanation is not incon- 

 sistent with the more general view that any condition tending 

 to lower the vitality may be responsible for a reversion to the 

 submerged type of leaf. 



In nature, the regression to the juvenile type of leaf some- 

 times occurs, not only in the case of an entire plant subjected 

 to adverse conditions, but also in the case of lateral shoots from 

 an individual which is otherwise producing the mature form of 

 leaf. Goebel 3 , for instance, examined an old example of Eichhor- 

 nia azurea (Pontederiaceae) which had wintered as a terrestrial 



1 Burns, G. P. (1904). 2 McCallum, W. B. (1902). 



3 Goebel, K. (1891-1893). 



