LITERATURE 445 



increased turgescence on the one side and a decreased turges- 

 cence on the other. 



In unicellular organisms, on the other hand, this mechanism 

 cannot be called upon ; and great difficulty has been met with 

 in explaining how an elongated cell, for instance, of a hypha, 

 can curve itself. Some authors have suggested that the 

 cell- wall on the convex side becomes excessively extensible, on 

 the concave side excessively rigid. Again, it has been sug- 

 gested that this change takes place rather in the protoplasm 

 inside the wall than within the wall itself. If, however, with 

 many plant physiologists we regard the cell- wall as the truly 

 living, only considerably modified, protoplasm, then we may 

 accept both of these views, and, uniting with them the expla- 

 nation advanced for phototropism in multicellular organs, say : 

 The curving of the unicellular organ is probably due to the 

 increased extensibility, gained through imbibition of water, of 

 the whole protoplasm (including cell-wall) of the convex side, 

 together with a corresponding diminution on the concave side. 

 In a word, then, the mechanism of phototropism may be stated 

 to be the excessively rapid imbibition of water by the proto- 

 plasm on the convex side. 



But what starts the mechanism ? This is the same question 

 that arises with other growth responses. Its answer must be 

 deferred to a later chapter. 



LITERATURE 



ADRIANOWSKY, A. '83. Wirkung des Lichtes auf das erste Keimungsstadium 

 der Samen. [Russian.] Abstr. in Bot. Centr. XIX, 73-75. 



ADUCCO, V. '89. Action de la lumiere 'sur la duree de la vie, la perte de 

 poids, la temperature et la quantite de glycogene hepatique et muscu- 

 laire chez les pigeons soumis au jeune. Arch. Ital. de Biol. XII. 

 208-214. 28 July, 1889. 



BARY, A. DE, '63. Recherches sur le developpement de quelques cham- 

 pignons parasites. Ann. d. Sci. Nat. (Bot.). (4), XX, 5-148. 



BECLARD, J. '58. Influence de la lumiere sur les animaux. Comp. Rend. 

 XL VI, 441-453. 



BERT, P. '72. Influence des divers rayons colore's sur la vegetation. C. R. 



Soc. Biol. Paris. XXIII, 67-69. 



'78. Influence de la lumiere sur les etres vivants. Revue Sci. (2), VII, 

 981-990. 20 Apr. 1878. 



