§■1] 



PHOTOTAXIS AND PHOTOPATHY 



191 



interval is a region of relatively high intensity (Fig. 56, 6) ; 

 in aquatic plants the epistrophic interval occurs in a region 

 of low intensity (Fig. 56, 1 and 2) ; and in shade-loving 

 aerophytes in an intermediate position (Fig. 56, 4 and 5). 

 One may say that every species is attuned to a certain intensity 

 and range of light, in which epistrophe occurs, just as in 

 swarm-spores there is a certain intensity and range of light in 

 which positive phototaxis occurs, and that attunement depends 

 upon the conditions to which the organism has adjusted itself 

 through living in them. 



— ^ + 



a. 



5. 



6. ■!;JE;S;.;':"";K:r 



Fig. 56. — A diagram of Mooke's photrum, showing for six spaces the epistrophic 

 interval (shaded region). 1. Anacharis (Elodea) canadensis, "water-weed." 

 2. Lemna trisulca. 3. Saxifraga granulata (position of positive critical point). 



4. Oxalis acetosella (position of negative critical point only approximate). 



5. Pteris critica (positive critical point). 6. Pyrethrum sinense (garden Chry- 

 santhemum). -{- indicates the brighter end of the photrum. 



Concerning the question of the mechanism of the movement 

 of the chlorophyll grains, there is much difference of opinion. 

 It is urged on the one hand that the chlorophyll grains move 

 actively to attain their new positions, and, on the other, that 

 they are passively carried by the cell currents. Of these two 

 views analogical reasons are perhaps the strongest for preferring 

 the second. 



As to the question in how far these movements can be 

 regarded as adaptive, we may say that Stahl believed that 

 they regulate the relation between intensity of sunlight and 

 assimilating area, so that the quantity of assimilation shall not 

 become too great. Hieeonymus ('92, p. 466) considers them to 

 be for the purpose of screening the nucleus. Mooeb (p. 222), 



