788 



When one considers that the rajs, which i)racticcilly rnn paiallel, 

 fall on the sporangiopliores, as on a cylindrical lens, then the conrse 

 of the rays in the body of the cell can be conceived as in fig. '2. 

 It will be seen that the illumination of the front and of the back 



of the cell necessarily must, differ 

 I'ather widely. There passes indeed 

 as much light through the front- 

 half as through the back lialf, 

 but by far the greater part of 

 the posterior wall the peripheral 

 proto[)lasm is more strongly illu- 

 Fig. 2. minated than that of the anterior 



wall ; in the middle of the posterior wall about twice as strongly 

 as in the middle of the anterior wall. With illumination which is 

 neither too strong nor too weak, the photo-growth reaction of both 

 sides commences at the same moment after about 3 min., but this 

 acceleration of growth continues at the end of the reaction somewhat 

 longer at the back than at the front and consequently after the cell 

 has shown a certain acceleration of growth, a curvature arises by 

 the action from that side, which is most strongly illuminated. 



The experiments concerning the absence of positive curvature and 

 occurrence of a negative one with large quantities of light, will be 

 continued in connection with results, to be obtained with the photo- 

 growth-reaction after omnilateral strong illumination. But in my 

 opinion the explanation of the positive phototropism in Phycomyces 

 has already been given b}" the above, and moreover I found in the 

 following facts new supports for this conclusion. 



I subsequently investigated what happens, when the unilateral 

 illumination is made weaker so that the lower limit of phototropism 

 is approached. We may now expect, according to the course of the 

 rays (see fig. 2) that a quantity of light would finally be reached, 

 which is too small to effect an increased stretching of the cell-wall 

 on the anterior side, but in consequence of refraction is just sufficient 

 to cause acceleration of growth on the posterior side. And that is 

 indeed what is found ! Table VI gives an example of the different 

 cases of this kind which I observed. 



In this way acceleration of growth was no longer observed ; at 

 the moment at which it would otherwise occur — that is after a 

 reaction-time of 5 — 7 min. with this very weak illumination — 

 the growth of the most strongly illuminated part of the cell-wall 

 only is accelerated and the only result is a curvature, which is weak 

 and often disappears again after a few minutes. 



