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THE AMEHICAN naturalist [Vol. LIIl 



mation from Brand, who estimated that a well-developed floating- 

 leaf of Nymphcea lutea was about eleven times the dry weight of 

 a submerged leaf of the same area. 



Another dicotyledon, Proserpinaca palustris, which was in- 

 vestigated by Burns, gave experimental results pointing to the 

 same general conclusion as those already quoted. The primitive 

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

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

 gardless of any external conditions which the experimenter could 

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

 summer gen'erally, almost every plant, w^hether growing in water 

 or air, produced the "land" type of leaf— the transition 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 con- 

 tent of the w^ater or contact stimulus. The only conclusion whicih 

 he holds to be justified 'by his experiments is that Proserpinaca 

 palustris has two forms, an adult form and a juvenile form; 

 under good vegetative conditions, it tends to produce the adult 

 form with the undivided leaf, the blossom and the fruit, while, if 

 the vegetative conditions are unfavorably influenced, a reversion 

 can be induced to the primitive form with the submerged type 

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

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

 terpretation is slightly different. He is inclined to regard the 

 occurrence of the water form as induced by the checking of trans- 

 piration and the increased amount of water w^hich hence accumu- 

 lates in the protoplasm. This explanation is not inconsistent 

 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 sometimes 

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

 conditions, but also in the case of lateral shoots from an individ- 

 ual which is otherwise producing the mature form of leaf. 

 Goebel for instance, examined an old example of Eichhornia 

 azurea (Pontederiaceae) which had wintered as a terrestrial plant 

 in a greenhouse ; the leaves were of the mature form, differen- 

 tiated into sheathing base, petiole and lamina, except in the case 

 of a lateral shoot, which, on the contrary, bore the grass like, 

 simple leaves which characterize the young plant. Goe'bel also 



