702 BIOLOGICAL EFFECTS OF RADIATION 



in determining the production and functional activity of antheridia and 

 oogonia in this species. 



Cultures of the liverwort Marchantia polymorpha L. were exposed 

 to various day lengths in experiments conducted by Wann (76). It 

 was found that this species responds to length of day in a similar manner 

 to that characteristic of long-day flowering plants. When subjected to 

 an artificially lengthened daily light period in winter, mature antheridio- 

 phores are produced in 3 to 4 weeks, mature archegoniophores in 6 to 

 8 weeks and mature sporophytes in 10 to 12 weeks. A relatively high 

 humidity tends to hasten the sexual response. 



Owing to its small size and simplicity of form, one of the aquatic 

 seed plants, Lemna major, will be treated here with these species. Clark 

 (12) has investigated the effects of intensity and daily duration of light 

 on reproduction. Under suitable nutrient conditions the rate of repro- 

 duction was directly proportional to the light period including continuous 

 illumination. At all day lengths reproduction was more rapid at 900 

 than at 400 foot-candles. Ashby (6) also investigated the interrelation- 

 ship of intensity and duration of light in the growth of Lemna. Colonies 

 were grown at constant temperature in a flowing nutrient solution with 

 day lengths of 6 and 12 hr. and continuous light, at intensities of 350, 

 700, and 1400 foot-candles. At lower intensities light added as "dura- 

 tion " or as intensity has the same effect on growth. Under the conditions 

 continuous light in all cases gave the highest growth rate. The rate of 

 growth increases with increase in intensity of light up to 700 foot-candles, 

 but falls off markedly at 1400 foot-candles. At all intensities the 

 highest constants of body weight occur with the 6-hr. light period and 

 the lowest with continuous light. 



INTERNAL CONDITIONS OF THE PLANT IN RELATION TO 



PHOTOPERIODISM 



Considerable research has been carried out on internal conditions of 

 the plant associated with the photoperiodic response, primarily for the 

 purpose of throwing light on the problem as to the mode of action of the 

 light period in influencing growth and development. While the data 

 obtained as a whole are of considerable interest in themselves, it must 

 be admitted that relatively little progress has been made in determining 

 the mechanism involved in photoperiodic response. In particular, no 

 adequate explanation has been developed as to why a long day favors 

 reproductive activity in one group of plants while a short day favors 

 reproduction in a second group. Lubimenko and Szeglova (42) consider 

 that a fundamental physiological distinction exists between the long-day 

 and short-day plants in that the specific ratio of the energy of respiratory 

 processes of oxidation to the photosynthetic processes of reduction is 

 greater in the former than in the latter. However, it is to be recalled 

 that frequently there is extensive storage of unused carbohydrate, with 



