BOOKS AND CURRENT LITERATURE 173 



portion of the spore wall it almost always bends very soon, so that its 

 tip becomes directed away from the source of light. The process of 

 germination is thus apparently related to differences in light intensity 

 on the different sides of the spore, and the young tubes are, at least 

 apparently, negatively phototropic. 



The evidence presented seems to the reviewer to indicate an orien- 

 tation of growth activity with reference to different intensities of the 

 impinging light received by different portions of the surface of the spore 

 or filament; these reactions are not apparently related directly to the 

 direction of the light propagation. This somewhat important distinc- 

 tion is not considered by the author. 



The purpose of this note is to emphasize the question that at once 

 arises to the reader, whether the reactions here described are to be con- 

 sidered as related to light at all. May they not be due to asymmetrical 

 heat or moisture conditions? Fromme's spores germinated on an 

 imbibed substratum (gelatin) exposed in a moist chamber, and a small 

 beam of diffuse daylight was allowed to fall upon the spores from one 

 side and from somewhat above the level of the substratum. It is 

 obvious that a light reaction may be involved in the behavior of the 

 germinating spores, but it seems just as obvious that moisture and 

 heat relations are here to be considered. The author does not take up 

 this consideration at all, though he does remark: "That positive 

 hydrotropism may' partially explain stomatal entrance [of such hyphae, 

 as germination occurs on leaves] is suggested by the work of Balls and 

 Fulton but it is doubtful that this can be a factor of primary importance." 



There appear to be about four logical possibilities here involved. 

 (1) Light intensity may be the controlling asymmetrical condition 

 and one-sided photochemical alterations within the cell may be the 

 internal control of the reaction. This seems to be the interpretation 

 adopted by the author. (2) The direction of the impinging light may 

 be the important external condition; as has been remarked, this seems 

 improbable from the data given, and it needs no more than mention 

 here. (3) A temperature difference between the shaded side of the 

 spore or filament and the illuminated side may bring about the ob- 

 served orientation. Here we meet with several possibilities. (3a) 

 The illuminated side receives more radiant energy and should therefore 

 be warmer. (3b) The air of the moist chamber may be warmer on 

 the illuminated side, so that that side of a cell may be warmed by 

 conduction. (3c) Convection currents in the air about the cell, brought 

 about by asymmetrical income of heat from the more distant surround- 



