THE VITAL PROPERTIES OF THE CELL 103 



reverse is the case when the temperature is lowered. In the first 

 case they also become more photophylic (light-seeking), and in 

 the latter- more photophobic (light-avoiding). 



" In addition, zoospores alter as regards their phototonus during 

 the course of their development, for they appear to be able to 

 withstand greater intensity when they are young than when they 

 are old." 



As is shown by the experiments of Cohn, Strasburger, and 

 Others, not all the rays of the spectrum are able to exert an 

 influence upon the direction of the movements of the spores, it 

 being only those which are strongly refracted (blue, indigo and violet) 

 that produce stimulation. 



If a vessel containing a deep-coloured solution of ammoniated 

 copper oxide, which only transmits blue or violet rays, be placed 

 between the source of light and the preparation, the spores are 

 seen to react just as if they came in contact with ordinary white 

 light ; on the other hand, they do not react at all to light which 

 has passed through bichromate of potassium solution, through the 

 yellow vapour of a sodium flame, or through ruby-red glass. 



Another very important and complex manifestation of the 

 effects due to light is seen in the movements of the chlorophyll , 

 corpuscles in plant cells. The light acts as a stimulus to proto- 

 plasm, which contains chlorophyll, causing the latter to collect by ' 

 means of slow movements in suitable places within the cellulose 

 membrane. 



The most suitable object for the study of these phenomena is the 

 Alga, Mesocarpus, upon which Stahl (IV. 34) has made some most 

 convincing observations. 



In the cylindrical cells, which are united together to form long 

 threads, a narrow band of chlorophyll is extended longitudinally 

 along the middle of the vacuole, which is thus divided into two 

 equal parts ; the ends of this band pass over into the protoplasmic 

 lining of the wall. Now this chlorophyll band changes its position 

 according to the direction of the impinging light. If it is exposed 

 directly from above or below to weak daylight, it turns its surface 

 towards the observer. If, however, on the contrary, it is arranged 

 so that only such rays as are parallel to the stage of the micro- 

 scope are allowed to reach the preparation from one side, the 

 green plates are seen to turn about through an angle of 90, so 

 that they take up an exactly vertical position, assuming now an 

 appearance of dark green longitudinal stripes, stretching them- 



