DIV. II 



PHYSIOLOGY 



331 



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

 (Fig. 273 JV), the advantage of which is not altogether clear. 



The form of the chlorophyll 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 protection 

 against too intense light, the chloroplasts of the 

 Siphoneae and Diatomeae (and the same thing is 

 observed in many plants) become balled together 

 in separate clumps. 



In correspondence with the changes in the 

 position of the chloroplasts, the colouring of 

 green organs naturally becomes modified. In 

 direct sunshine they appear lighter, in diffused 

 light a darker green. 



2. Chemotaxis ( 94 ) 



Chemotaxis results, as mentioned 

 above, from the unequal distribution 

 of substances dissolved in water. Posi- 

 tive chemotaxis leads to the irritable 

 plants accumulating in the region of 

 higher concentration of the chemotactic 

 material. 



Such substances are of definite 

 nature. Thus, for example, many bac- 

 teria are "attracted" by particular 

 organic or inorganic food materials, 

 e.g. peptone, sugar, meat-extract, phos- 

 phates, etc., while they are " repelled " 

 by other substances such as acids and 

 alkalies. While the chemotaxis here 

 serves the process of nutrition, its use 

 is different in the case of spermatozoids ; 

 these male sexual cells are thus attracted 

 to the egg -cells. Nuclei and chloro- 

 plasts may also show chemotactic move- 

 ments. 



The spermatozoids of the Ferns are attracted by malic acid or malates to the 

 neck of the archegonium ; in the case of the spermatozoids of Lycopodium, citric 

 acid, in Mosses, cane sugar solution, and in the Marchantieae proteid substances 

 are the respective attractive substances. Often extremely minute quantities of the 

 substance will bring about active irritable movements ; thus even a O'OOl per cent 

 solution of malic acid will attract the numerous spermatozoids of a Fern swimming 

 in pure water. In chemotaxis as in phototaxis we can distinguish phobic and topic 

 modes of reaction. 



Aerotaxis determined by oxygen is found in the case of Bacteria ; 



PIG. 273. Varying positions taken by the 

 chlorophyll grains in the cells of 

 Lemna trisulca in illumination of differ- 

 ent intensity. T, in diffuse daylight ; 

 S, in direct sunlight; N, at night. 

 Tho arrows indicate the direction of 

 the light. (After STAHL.) 



