PHOTOPERIODISM 483 



periodically controlled, but the particular light requirement for each one 

 is different. Preceding the molts, sometimes by as much as 2 months, 

 physiological changes occur in the skin, and these determine the hair 

 type and color at the next molt. The physiological condition of the 

 skin at a certain time is determined by plucking a small area on the 

 animal. New hair covers the plucked area, and its color is determined 

 by the physiological stage reached by the skin at the time of plucking. 

 During the first autumn molt, or very shortly thereafter, the skin changes 

 to the physiologically white condition, and many weeks before the spring 

 molt it again shifts from physiologically white to brown. Lyman found 

 that artificially lengthened photoperiods in autumn prevented the change 

 of skin condition from brown to white, or if the change had already 

 occurred, the long-day treatment resulted in its reversal to the brown 

 condition. 



Although photoperiodism controls sexual reproduction in a great num- 

 ber of animals, the first evidence of its effects varies. In certain aphids 

 (Markovitch, 1924) the proper treatment results in production of sexual 

 individuals instead of asexual ones. Certain birds and mammals respond 

 to favorable treatment by a rapid increase in size of the gonads and by 

 changes in their structure leading to the production of gametes. The 

 first obvious effect in the snail (Jenner, 1951) is to promote ovulation. 

 The gonad apparently contains fully formed eggs at all times, once the 

 snail has attained a certain size, and the function of light is to cause 

 their release. Proper day length results in earlier implantation of the 

 embryo in some animals, such as the martin (Pearson and Enders, 1944), 

 and thus in reduction of the period of gestation by several weeks. 

 Despite these apparent differences in response of widely different species, 

 it is not unlikely that the same initial light action is responsible for each. 



ORGAN OF PERCEPTION 



In plants the leaf is effective in the photoreaction controlling flower- 

 ing and other morphogenic responses (Cajlachjan, 1936; Knott, 1934; 

 Moskov, 1936). Dark periods favorable to flowering are effective when 

 applied to a single leaf or part of a leaf of certain short-day plants, e.g., 

 Soja max var. Biloxi (Glycine max) (Borthwick and Parker, 1938), 

 Xanthium pensylvanicum (Hamner and Bonner, 1938), and Kalanchoe 

 hlossfeldiana (Harder, 1944, 1948). A leaf is most effective as soon as it 

 is fully expanded. Presence of other leaves subject to light conditions 

 noninductive for flowering apparently is not inhibitory except as might 

 result from their effect on translocation. It does not follow that the stem 

 and other parts are not also effective, particularly if the leaf is vestigial, 

 as in Asparagus officinalis and species of cacti. Direct irradiation or 

 darkening of the terminal meristem does not control floral initiation 



