302 



BOTANY 



i'ART I 



the directiou of the rays of liglit ; it then proceeds straight on in this direction 

 (topic reaction). In otlier cases a qnite different mode of reaction is observed 

 (pliobic reaction) ; the organism bounds back when its movements have brought it 

 into a region of the medium where it is exposed to a distinctly different intensity 



of light. No rotation of the organism takes 



place in this case. 



A very striking example of photo- 

 taxis is aflbrded by the chloroplasts 

 within the cell {^■'). These movements 

 have the result of bringing the chloro- 

 phyll grain into such a position that 

 it can obtain an optimal amount of 

 light. This object is sometimes at- 

 tained by rotation of the chloroplast, 

 and sometimes by its movement to 

 another position in the cell. 



In the cylindrical cells of the tilamentous 

 Alga Mcsocarpus, 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 illumin- 

 ated (transverse position) ; when, on the other 

 hand, they are exposed to direct 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 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 rela- 

 tively to the cell-walls. In moderate light 

 the chlorophyll bodies are crowded along the 

 walls, which are at right angles to the direc- 

 tion of the rays of light (Fig. 227 T). They 

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

 light becomes too intense, and so retreat as far as possible from its action (Fig. 227 ^')- 

 In darkness or in weak light the chloroplasts group themselves in still a third way 

 (Fig. 227 N), the advantage of which is not altogether clear. 



The form of the chloroi>hyll grains themselves undergoes modification during 

 changes in their illumination ; in moderate light they become flattened, while in 

 light of greater intensity they are smaller and thicker. As a special mode of pro- 

 tection against too intense light, the chloroplasts of the Siphoneao and Diatomeae (and 

 the same thing is observed in many plants) become balled together in separate clumps. 



Fig. 2'27. — Varying positions taken by the 

 chloropliyll grains in tlie cells of 

 Lemna trisulca in illumination of dift'er- 

 ent intensity. T, in diffuse daylight ; 

 S, in direct sunlight ; N, at night. 

 Tlio arrows inrlicate the direction of 

 the light. (After Stahl.) 



