GROWTH AND MOVEMENT 



437 



Light and form. The form of the aerial parts of most plants is pro- 

 foundly influenced by light, directly or indirectly. This is shown by the 

 striking changes that ensue (etiolation) when they are 

 grown in darkness. Without starvation this is pos- 

 sible only with -plants that have already stored a 

 sufficient amount of surplus food. One who has 

 observed the long pallid shoots of a potato which 

 has sprouted in the dark will have seen the general 

 effects. The stems tend to elongate much more than 

 usual, though they are not necessarily more slender; 

 the branching is at a different angle; and the leaves 

 remain small and imperfectly developed. (The pallor 

 from lack of chlorophyll and the presence of carotin 

 are features already mentioned.) On the whole, 

 elongation is likely to be accentuated, breadth is likely 

 to be repressed (fig. 676). Though these are the 

 common results of the lack of light during develop- 

 ment, they are by no means universal. Thus, there 

 are plants whose stems do not elongate, and others 

 whose leaves are not reduced. But if not these, 

 other characteristics may be altered; e.g. reduction 

 of the mechanical elements of the tissues is one of 

 the less obvious effects. Scarcely a plant escapes but 

 those that pass all their lives in darkness, and only 

 those parts that are buried in the soil are exempt from 

 the formative influence of light. 



Dorsiventrality. In plant organs not grown in 

 darkness, but of which one side is better illumi- 

 nated than the other, light effects can be observed. 

 One effect is the development of a distinctly different 

 structure in the better lighted surface as compared 

 with the shaded one, and since these are naturally the 

 upper and under surfaces, an organ showing such 

 differences is termed dorsiventral. 1 Thus the pali- 

 sade portion of the mesophyll of leaves owes its exist- 

 ence chiefly to light. 2 Dorsiventrality in the liverworts is likewise due 

 mainly to light. None shows this better than the common Marchantia, 



FIG. 676. Plant 

 of Phaseolus grown 

 in darkness. After 



MACDOUGAL. 



1 Dorsiventral organs may owe the difference of their faces to other formative stimuli, 

 e.g. to gravity. See footnote, p. 421. 



