l6o OUTLINES OF PLANT LIFE. 



Those conditions which act more generally and uniformly upon 

 a large number of plants serve to determine the form and 

 mode of development of members. 



221. Light. — The effect of light on growth is different in 

 different plants and even in different members of the same 

 plant. In general light retards growth in length. Stems 

 grown in darkness usually become excessively elongated. 

 Those which under normal illumination have very short in- 

 ternodes, in diminished light may have them well developed, 

 as occurs, for example, in dandelions growing in deep shade. 



In general, light accelerates the growth of leaves in area. 

 Leaves of shoots grown in darkness remain small. 



Light affects not only the external form but the internal 

 structure. The difference in structure between the upper and 

 lower surface of the thallus of Marchantia (*\ 52, fig. 38) and 

 of the leaves of higher plants (fig. 106) is due to the greater 

 illumination of the upper surface. In diminished light cell 

 walls may not thicken normally, and mechanical tissues are 

 weakened. "Lodging" of oats and such grasses is mainly 

 due to this cause (fig. 120.) 



222. light and temperature. — The combined variation 

 of light and temperature between day and night establishes a 

 daily period in the growth of all plants. The withdrawal of 

 light at night permits an increase in the rate of growth in 

 length, which reaches its maximum in some plants shortly 

 after midnight, in others not until the early morning. During 

 the day its retarding effect diminishes the rate of growth, 

 which reaches a minimum some time in the afternoon. The 

 minor fluctuations in temperature, as well as the generally 

 higher temperature during the day and lower during the night, 

 introduce variations in the rate of growth, which obscure, but 

 do not counteract, the retarding influence of light. (See fig. 

 121). This daily period is so impressed upon the constitu- 

 tion of the plant that it maintains it for a considerable time 



