rnOTOPERIODISM 701 



affected in much the same way. The injurious effects of the intermediate 

 alternations of Ught and darkness inckided destruction of chlorophyll, 

 localized dying of the leaf tissue, reduced leaf development, etiolation, 

 and decrease in stem growth and in production of dry matter. When the 

 duration of the light periods was reduced to one-half that of the periods 

 of darkness, these injurious effects were accentuated and when the 

 light intervals were correspondingly increased the condition of the 

 plants was improved. 



THE PHOTOPERIODIC RESPONSE AND HEREDITY 



In 1919, before photoperiodic response had been recognized as a 

 phenomenon of wide occurrence, AUard (3) made a study of the inherit- 

 ance of the indeterminate or nonflowering type of growth exhibited by 

 the Maryland Mammoth variety of tobacco {Nicotiana Tahacum) when 

 exposed to a long day. It was found that in crosses with ordinary 

 varieties, which readily flower in either long- or short-day lengths, the 

 nonflowering character is recessive and Fi plants readily flower in a 

 long day. In the F^ generation the nonflowering or mammoth plants 

 occur in proportions approaching the simple Mendelian ratio of 25 per 

 cent. Bremer (11) has reported observations on inheritance of day-length 

 response in lettuce, the winter or spring and the summer sorts of which 

 respond differently to length of day. The former develop no heads in 

 the long days of summer but, on the contrary, at once produce flowering 

 stems. The summer sorts are relatively indifferent to day length, form- 

 ing heads in both long and short days. Bremer showed that this contrast 

 in response rests on a simple Mendelian character. The allelomorph 

 "formation of flowering stem is linked with day length" is dominant over 

 the allelomorph "formation of flowering stem is not linked with day 

 length." Further work will be required to determine to what extent 

 such simple inheritance relations apply to other species and to other 

 features of photoperiodism. 



PHOTOPERIODISM IN THE LOWER GREEN PLANTS 



Research has shown that some of the lower green plants are capable 

 of responding to differences in the Ught period to a degree comparable 

 with the effects on the higher plants. Karling (33) has reported results 

 obtained with the aquatic, Chara fragilis Desvaux exposed to the winter 

 daylight alone and supplemented with electric light at night. Under 

 the conditions of the experiments length of day appears to be a primary 

 factor in inducing the formation of antheridia and oogonia, the supple- 

 mentary illumination of very low intensity effecting rapid response in 

 midwinter, while in nature fruiting occurs only in summer and early 

 fall. Under the conditions temperature, within the minimum and 

 maximum hmits for vegetative growth, apparently is a secondary factor 



