206 BOOKS AND CURRENT LITERATURE 



tion after death, many current conceptions seem to require revision. 

 If the cell wall proves to be effective in this case, as it appears to be in 

 some seeds, inquiry into the precise character of cell walls which may 

 thus act differentially would be of value. 



On the other hand it is possible that Brown's xylene experiment 

 may not deal with the actual mechanism of the living leaf, but may 

 be concerned wholly with imbibition changes, in which a heavy-walled 

 tissue shrinks more than an opposing thin-walled tissue, upon being 

 transferred from a liquid that promotes swelling of cell walls (water) 

 to one in which there is little swelling (xylene). Such tissues are pres- 

 ent in Dioncea leaves and in the pulvini of Mimosa and occupy positions 

 which might suggest such explanation. 



Speculations such as these indicate the diversity of the problems 

 presented by this paper and, while it is true that even the main fea- 

 tures of the mechanics of the reaction in Dioncea can not be considered 

 as settled as yet, Brown's work ought to stimulate investigation and it 

 surely indicates some of the main lines along which further study may 

 be profitable. 



The second part of the paper deals with the relation between the 

 frequencj^ of equal, mechanical stimuli and the total time elapsing 

 before the beginning and the end of leaf-closure. The results are ex- 

 pressed in the form of a graph which indicates that when the stimuli 

 are applied at short intervals the number needed before closure is com- 

 plete is relatively greater than when they are applied less frequently. 



Hooker's paper on the tentacles of Drosera also records permanent 

 enlargement of the active tissue during response, one side elongating 

 during bending and the opposite side during recovery. The reaction 

 may be repeated if a day without stimulation intervenes, but no more 

 than three reactions could be evoked from the same tentacle. It is 

 apparent from Brown's data that the same cells do not share equally 

 in the successive reactions of Dioncea, and Hooker finds that the same 

 portion of the tentacle does not respond in subsequent reactions. 

 Drosera tentacles remained curved after plasmolysis had occurred and 

 the permanent alteration of their cell walls is thus apparently accom- 

 plished more quickly in this case than in that of Dioncea, although there 

 is nothing to indicate that the series of processes in the former may not 

 include the intermediate stage observed by Brown. It is surely time 

 that the various processes generally included under the term "growth" 

 should be considered separately and that this word be no longer used 

 to indicate a single "vital process." The two papers here reviewed 



