Light 311 



Whether light produces its effect on cell size by changing osmotic 

 concentration, permeability of cell membranes, attraction of protoplasm 

 for water, character of the cell wall, or other processes is not clear. It is 

 significant that not merely is cell size increased in low light intensity but 

 size along the polar axis of the cell. 



Meier (1934) studied the effects of the intensity of light on cell di- 

 vision in the unicellular alga Stichococcus bacillaris and found that its 

 multiplication in culture is proportional to the intensity of illumination 

 up to a certain point but that high intensities check it. 



Thomson (1954) grew seedlings of oats and of peas with different 

 amounts of light and reports that light accelerates whatever growth 

 processes are going on while it acts, its effect depending on the stage 

 of development of the tissues concerned. Exposure early in the course of 

 either the cell-division phase or that of cell elongation hastens the 



Fig. 13-2. Light and vascular development. Portion of the vascular cylinder of the 

 stem of Vicia faba grown in darkness (left) and in the light (right). (From Borg- 

 strom. ) 



completion of that phase of growth, but after it is under way light 

 hastens the transition to the next phase and thus reduces the final num- 

 ber or length of cells. 



Shape traits may be modified by light. Smirnov and Zhelochovtsev 

 (1931) found that in Tropaeolum leaves the reduction of blade expan- 

 sion in weak light modified the fundamental growth pattern. Njoku 

 (1956a) reports that differences in light intensity change the leaf shape 

 in Ipomoea. 



Anatomical characters are also affected. Penfound ( 1931 ) observed 

 that stems of Helianthus and Polygonum growing in full sunlight have a 

 much greater amount of xylem and more and thicker-walled cells in the 

 mechanical tissue than those in shade. The much reduced vascular tissue 

 of etiolated plants is well known (Fig. 13-2). Bond and others have 

 found that as light intensity is reduced the development of the endo- 

 dermis increases in the stem, where it normally is weak or absent. That 



