330 BOTANY PAKT i 



the former case the taxis is positive, and in the latter negative. The 

 nature of the reaction frequently depends not only on the object, but 

 on the external conditions. 



1. Phototaxis ( 92 ) 



Phototactic movements may be best observed when a glass vessel 

 containing water in which are Volvocineae, Chlamydomonadinae, or 

 swarm-spores of Algae is exposed to one-sided illumination from a 

 window. After a short time the uniform green tint of the water 

 disappears, since the motile organisms have all accumulated at the 

 better-illuminated side of the vessel. If the latter is turned through 

 an angle of 180 the Algae hasten to the side which is now illuminated. 

 If, however, a stronger light, such as direct sunlight, is allowed to fall 

 on the vessel the same organisms which till now have reacted positively 

 become negatively phototactic and swim away from the source of light. 

 Other external factors may have the same effect. 



In some organisms, such as the plasmodia of Myxomycetes, we find a negative 

 reaction even to a light of low intensity. There are also colourless organisms 

 which have a positive phototactic reaction. In nature phototactic movements 

 usually bring the organism into a position of optimal illumination. 



There are two distinct kinds of phototaxis. In the one (TOPOPHOTOTAXIS) 

 the organism places itself in the direction of the rays of light, and moves towards 

 or away from the source of light. In other cases (PHOBOPHOTOTAXIS) the organism 

 reacts on the passage from light to darkness by a sudden movement that brings it 

 back into the light ; it thus remains fixed in the illuminated spot. 



A very striking example of phototaxis is afforded by the chloro- 

 plasts within the cell ( 93 ). These movements have the result of bringing 

 the chlorophyll grain into such a position that it can obtain an optimal 

 amount of light. This object is sometimes attained by rotation of the 

 chloroplast, and sometimes by its movement to another position in 

 the cell. 



In the cylindrical cells of the filamentous Alga Mesocarpus, the chloroplasts, in 

 the form of a single plate suspended length -wise in each cell, turn upon their 

 longitudinal axes according to the direction and intensity of the light. In light of 

 moderate intensity they place themselves transversely to the source of light, so that 

 they are fully illuminated (transverse position) ; when, on the other hand, they are 

 exposed to dii-ect sunlight, the chlorophyll plates are so turned that their edges 

 are directed towards the source of light (profile position). 



In the leaves of mosses and of the higher plants and in fern prothalli a similar 

 protection of the chloroplasts against too intense light, and their direct exposure, 

 on the other hand, to moderate illumination, is accomplished, where they are of 

 a different form and more numerous, by their different disposition relatively to the 

 cell walls. In moderate light the chlorophyH bodies are crowded along the walls, 

 which are at right angles to the direction of the rays of light (Fig. 273 T). They, 

 however, quickly pass over to the walls parallel to the rays of light as soon as the 

 light becomes too intense, and so retreat as far as possible from its action (Fig. 273 S). 



