THE VITAL PROPERTIES OF THE CELL 99 



Then during the spring, as the temperature rises, they move in an * 

 opposite direction, ascending to the warmer superficial layers. 



II. Light Stimuli. In many cases light, like heat, acts as a 

 stimulus to animal and plant protoplasm. It induces character- 

 istic changes of form in individual cells, and causes movements in 

 fixed directions in free-living unicellular organisms. Botanists 

 have obtained especially interesting results in this department. 



The plasmodia of JEthalium septicum only spread themselves 

 out on the surface of the tan in the dark ; in the presence of light 

 they sink down below the surface. If a small pencil of light is 

 allowed to fall upon a plasmodinm which has spread its network 

 upon a glass slide, the protoplasm is immediately seen to stream 

 away from the illuminated portion, and to collect in the parts 

 which are in shadow (Barenezki, Stahl IV. 35). 



Pelomyxa palustris, an organism resembling the Amoeba, is 

 actively motile in shadow, extending and protruding broad 

 pseudopodia. If a fairly powerful ray of light impinges upon it, - 

 it suddenly draws in all its pseudopodia, and transforms itself into 

 a globular body. Only after it has rested quietly in the shade 

 for a time does it gradually recommence its amoeboid movements. 

 " If, on the other hand, daylight is admitted gradually during a 

 period of rather less than a quarter of an hour, no effects of stimu- 

 lation are to be perceived ; this is also the case when, after a 

 prolonged illumination, the light is suddenly withdrawn" (Engel- 

 mann IV. 6 b). 



The star-shaped pigment cells of many invertebrates and verte- 

 brates, which have been described under the name of chromatn- 

 phores (IV. 3, 29, 30, 33), react very actively to light ; they are 

 the cause of the changes of colour so often seen in many Fishes, 

 Amphibians, Reptiles, and Cephalopods. For example, the skin of 

 a Frog assumes a lighter shade of colour when under the influence 

 of light. This is due to the fact that the light causes the black 

 pigment cells, which extend their numerous processes through the ^ 

 thick skin, to contract up into small black points. In addition, as , 

 they become less prominent, the green and yellow pigment cells, 

 which do not contract, become more easily seen. 



Further, the pigment cells of the retina become considerably / 

 altered in form under the influence of light, both in vertebrates 

 (Boll) and in invertebrates, for instance in the eyes of Cephalopoda 

 (Rawitz IV. 31). 



It is a well-known fact that many unicellular organisms which 



