Light 309 



INTENSITY OF LIGHT 



Since rate of photosynthesis increases with light intensity, up to a cer- 

 tain point, the growth and vigor of a plant are generally proportional, 

 within limits, to the brightness of the light to which it is exposed. 

 Shirley (1929) grew a variety of plants in different intensities of day- 

 light and found that at low ones dry weight was directly proportional 

 to intensitv but that at higher ones growth was relatively less. He ob- 

 served that intensity also affected certain qualitative traits, such as ratio 

 of root to shoot, strength of stem, thickness of leaves, and development 

 of vascular tissue. 



It has frequently been observed that plants grown in shade have rela- 

 tively small root systems. In general it may be said that stem elongation 

 varies inversely with light intensity but that width varies with it directly 

 ( Popp, 1926 ) . The effect of light may be different on different parts of 

 the plant and at different stages of development. Some morphological 

 effects of light may be quite specific though the mechanisms involved 

 are unknown. Some herbaceous stems, for example, have zig-zag form 

 in the light but are straight if grown in the dark. Plants that twine in the 

 light usually lose this ability in darkness. 



Light is also important morphogenetically for some plants lacking 

 chlorophyll. In certain mushrooms, for example, the fruiting body will 

 not develop normally in complete darkness although the whole vegetative 

 mycelium is subterranean (Borriss, 1934b). 



Etiolation. The most conspicuous effect of differences in light intensity 

 on plant structure is to be seen in the phenomenon of etiolation. It is a 

 familiar fact that green plants growing in darkness or relatively weak 

 light tend to be tall and spindly, with small, pale leaves, weak roots, and 

 poorly developed vascular tissues. Such plants soon die unless consider- 

 able reserve food is available in seeds or other storage organs, in which 

 case etiolated growth may continue for some time. The early work of 

 Kraus ( 1869 ) showed that etiolation involves a considerable increase 

 in cell length, though in most cases this is accompanied by some increase 

 in cell number in the longitudinal dimension. 



Different parts of the plant and different species differ considerably 

 in their manifestations of etiolation. Only shoots etiolate and not roots, 

 flowers, or fruits. Avery, Burkholder, and Creighton ( 1937a ) observed 

 marked differences between the first internode and the coleoptile as to 

 their elongation in various light intensities. Intensity may also affect the 

 proportions of parts. In Tropaeolum plants, for example, which are grow- 

 ing in weak light the ratio of petiole length to lamina width becomes 

 progressively greater as the leaves develop, whereas under normal 



