PHOTGMORPHIC ACTIONS 85 



plants, for in darkness certain fungi either cease to form particular organs, 

 or these assume abnormal shapes. 



Exposure to light is not a general condition for growth, and hence 

 even in chlorophyllous plants certain organs may be withdrawn from it. 

 Thus subterranean roots mostly grow in complete darkness, and little or no 

 light reaches the inner living tissues of thick or corky aerial organs. As 

 might be expected, the formative changes produced by light are most 

 pronounced in the case of those organs which -utilize it as a source of 

 energy, but all organs which normally carry out their functions above the 

 substratum may exhibit a power of photomorphic response. It is in 

 accordance with the special character and distribution of the light-stimulus 

 that it should be a most important directive and formative agency, and that 

 it should on the whole induce more pronounced growth-reactions than the 

 more generally distributed factor of temperature. 



In darkness a plant continues to grow normally so long as it remains 

 in a condition of phototonus, i. e. so long as the influence of the previous 

 exposure persists. This period is frequently very prolonged, so that 

 exposure to darkness exercises no such direct retarding influence upon 

 growth as does a low temperature. If the plant is kept for a sufficient 

 length of time in darkness for its phototonus to be lost, it either falls into 

 a condition of dark-rigor and ceases to grow, or its growth becomes 

 abnormal in character. This may take place even when an ample supply 

 of food is assured, but in green plants the effect produced may be 

 increased as the result of starvation. As a general rule, however, growth 

 appears to be somewhat quickened in darkness, and retarded by exposure 

 to light, so long as the exposures are not too prolonged, and the plant or 

 organ is well nourished and is in a growing condition. 



The direct and indirect influences exerted by light and by different 

 light-rays need to be distinguished as far as possible from one another. 

 We are at present concerned more especially with the stimulatory action 

 of light, and shall only incidentally discuss results due to the production 

 or non-production of organic food. Insolation always produces a rise of 

 temperature, and usually also increases the activity of transpiration, but 

 these subjects have already been dealt with. The curvatures due to 

 heliotropism will form the subject of a later chapter 1 . 



SECTION 24. Photomorphic Actions. 



The shapes of certain plants are little or not at all affected by the 

 presence or absence of light, while in other cases pronounced differences 



1 For the influence of light on the distribution of plants see Schimper, Pflanzengeographie, 

 1898, p. 61 ; and on that of algae cf. Berthold, Report of the Naples Zool. Station, 1882, Vol. Ill, 

 P- 393 ; Oltmanns, Jahrb. f. wiss. Bot., 1891, Bd. xxiil, p. 416. 



