1 84 



PLANT LIFE. 



to bring about permanently unequal growth in a certain part, 

 or to cause one part to grow more or less rapidly for a time 

 than another. Such variations in growth produce curvatures 

 in the parts concerned and move members connected with 

 them. They are therefore discussed in the chapter on Move- 

 ments. Those conditions which act more generally and 

 uniformly upon a large number of plants have a tonic effect 

 and serve to determine the form and mode of development of 

 members. 



262. Light. — The tonic effect of light is different upon 

 different plants and even different members of the same plant. 



I 



Fig. 185.— Part of the transverse sections of the stem of rye. A, from a plant grown 

 fully exposed to light; B, from a "laid" plant imperfectly exposed to light. a t 

 epidermis ; b, c, mechanical tissues ; d, thin-walled tissues. Highly magnified.— After 

 Koch. 



In general light retards growth in length. Stems grown in 

 darkness usually become excessively elongated. Those 

 which under normal illumination have internodes very 



