PHOTOPERIODISM 687 



flexuous and may form loops. Under the length-of-day gradient from 

 August to November a large percentage of completely neutral, vestigial 

 tassels can be developed in a population. With the decreasing day 

 length of autumn femaleness is expressed only at the base of the tassel, 

 the middle region is staminate, and the tip is always neuter. With the 

 increasing day length of late winter and early spring femaleness also 

 may be expressed in the tip and branches. Tournois (75) described 

 numerous abnormalities in reproductive structures of hemp associated 

 with precocious flowering in response to the short days of early 

 spring. 



In a study of the influence of light on the unfolding of the flower bud 

 in Hedera Helix, which behaves as a short-day plant, Sigmond (68) found 

 that the processes involved are rhythmic in character and are facilitated 

 by the alternation of day and night, so that night illumination has a 

 retarding effect. Light has no direct stimulating action on the opening 

 of the flower. 



GROWTH RELATIONS 



The fact that entrance upon reproductive activity usually checks or 

 abruptly terminates growth in the plant makes it somewhat difficult to 

 obtain a clear picture of growth relations as affected, on the one hand, by 

 light periods which tend to initiate reproduction and, on the other hand, 

 by periods which favor only vegetative activity. In general, growth 

 characteristics of the short-day type of plant with light periods below 

 the critical rather closely resemble those of the long-day type of plant 

 with hght periods above the critical. In both cases the rate of growth, 

 the height attained, and the production of dry matter as a rule tend to 

 increase with increase in duration of light, but there are certain plants 

 which are injured by a very long daily light period and there are still 

 others which grow best even with a moderately short light period. 

 Similar growth characteristics also will apply to the indeterminate group 

 of plants with all hght periods, since in this case flowering is always 

 involved. The situation is different where light periods tending to 

 suppress reproduction come into play. With the short-day plant there 

 is, as a rule, comparatively little change in the general type of growth, 

 except as to absence of flowering and fruiting, but in the long-day type 

 important morphological changes usually appear and total production 

 of dry matter affords about the only simple basis for comparison. Since 

 vegetative activity usually will continue at a rapid rate for a prolonged 

 period in short-day plants exposed to day lengths above the critical, the 

 final height and the plant mass may greatly exceed those of plants exposed 

 to shorter day lengths which induce early flowering. This may hold 

 true, but to a much lesser extent, for the long-day plants exposed to light 

 periods too short to induce flowering. 



