314 Morphogenetic Factors 



The relation of wave length to auxin production and other problems 

 of photomorphogenesis have been discussed by Stolwijk (1954). 



Other developmental traits are affected by light quality. Thus Funke 

 (1931) observed that in heterophyllous water plants, where the juvenile 

 immersed leaves are ribbon-like, these never develop into anything else if 

 the plants are grown in red or in green light. In blue or white, however, 

 normal mature foliage is produced. This change may be reversed by 

 changing the wave length of the light. In root cultures of peas, red light 

 inhibits the formation of lateral roots more effectively than blue or 

 green, perhaps by inactivating substances necessary for root growth 

 (Torrey, 1952). Many other instances of the effects of light of different 

 wave length on development in higher plants have been reported. 



Less work has been done with lower plants. Meier (1936), again with 

 the alga Stichococcus, found that in cell culture the individual cells in a 

 given time multiplied fourfold in white light, threefold in blue, but 

 only twofold in yellow and red. Green light proved to be destructive to 



them. 



In the slime mold Didynium nigripes, light is necessary for the develop- 

 ment of sporangia (Straub, 1954). Green light has no effect but red 

 and blue have. If plasmodia treated with these wave lengths are killed 

 by freezing and fed to living plasmodia, the latter produce sporangia 

 after a briefer exposure to light and much more rapidly than control 

 plasmodia which had been fed untreated ones. Evidently a substance con- 

 ducive to sporangium production is formed by the action of light of cer- 

 tain wave lengths. Gray (1953), using the slime mold Physarum poly- 

 cephalum, found that continuous irradiation with monochromatic light 

 in the blue and green and a narrow band in the yellow induced fruiting 

 bodies, the rate of their formation being inverse to the wave length of the 

 light used. He suggests that a changed acidity resulting from the irradia- 

 tion may be responsible for the production of sporangia. 



There are general discussions of the morphogenetic effects of different 

 wave lengths of light by Parker and Borthwick (1950) and Wassink and 

 Stolwijk (1956). 



DURATION OF LIGHT 



One might expect that the longer a plant is exposed to light favorable 

 for photosynthesis, the more it would grow and the more vigorous it 

 would be. Keeping plants in continuous light, however, is often found to 

 result in less vigor and in a disturbance of the normal reproductive 

 cycle. It is evident that the production of flowers and fruits is not some- 

 thing that inevitably occurs but rather that it will happen only when 

 environmental factors are favorable for it. In 1920 Garner and Allard pub- 



