322 Morphogenetic Factors 



rooted leaves respond to day-length by the same changes in form and 

 structure as do those that remain attached to the plant ( Schwabe, 1958 ) . 



In many woody plants studied, both flowering and vegetative growth 

 are markedly affected by the photoperiod (Wareing, 1956; Downs and 

 Borthwick, 1956). In general, short days induce dormancy and long ones 

 prolong growth. Marked ecotypes as to photoperiod have been found in 

 a number of species. 



Cellular characters are also affected. Von Witsch and Fliigel (1952) 

 found that in leaves of Kalanchoe Blossfeldiana ( 2n = 34 ) formed in long 

 days the mesophyll cells have chromosome numbers between 128 and 

 135. Under short days these cells are much larger and the degree of 

 polyploidy is increased, the chromosome number going up to about 540. 

 In tetraploid plants of Hyoscyamus niger produced by colchicine, the 

 critical day-length for flowering and the time preceding the elongation 

 of the internodes were both shortened, the number of leaves was re- 

 duced, and the time of flowering delayed, as compared with diploid 

 plants (Lang, 1947). 



The problems of photoperiodism are complicated by the fact, empha- 

 sized by Biinning ( 1956 ) and others, that there are endogenous rhythms 

 in certain of the physiological processes of the plant. It has been found, 

 for example, that a light period of 12 hours alternating with 12 hours of 

 darkness gives in many plants a different result from an alternation of 6 

 hours of light and 6 hours of darkness and thus two cycles in a day. The 

 total amount of light and darkness are the same but their effects are not. 

 There is evidently a changing sensitivity in the reaction of the plant 

 during the day to various environmental factors. This fact is of much 

 importance for plant physiology but its significance for morphogenesis 

 has as yet not been very fully considered. The existence of innate rhythms 

 may account for the conflicting results obtained in experiments on the 

 morphogenetic effects of various environmental factors. 



RELATION TO OTHER FACTORS 



The various morphogenetic effects of light provide an excellent example 

 of the complexity of interaction of factors in plant development. Light 

 powerfully influences flowering, but so do temperature, growth sub- 

 stances, nutrition, and genes, and in some cases water supply and gravity. 

 They often have parallel effects on vegetative structures as well. These 

 factors frequently can be interchanged to some extent and produce the 

 same result, as when temperature is substituted for photoperiod, and 

 vice versa. Auxin is closely concerned with many of the traits that light 

 affects, but the exact relation between light and auxin is not clear. Some- 

 times light seems to destroy it and sometimes to stimulate its produc- 



