6o6 FACTORS AFFECTING GROWTH 



for an indefinite period. Similar localized reactions of plants to the length of 

 the light period can be demonstrated in other species. 



The influence of the duration of the daily photoperiod upon reproductive- 

 ness is by far the most outstanding, but by no means the only effect of this 

 climatic factor on growth. Some of its other more important effects are 

 upon: (i) production of storage organs (Zimmerman and Hitchcock, 1929), 

 (2) sexual expression (Schaffner, 1923, 1930), and (3) rejuvenation (Garner 

 and Allard, 1923). 



Species native to arctic regions, as w^ell as a number of crop species (toma- 

 toes, grains, berries) develop vv^ell under summer conditions of continuous 

 or nearly continuous daylight. In general development of such plants at 

 high latitudes takes place more rapidly than in regions of shorter summer 

 day-lengths. Harvey (1922) and others have shown that a number of annual 

 species such as wheat, oats, flax, cotton, buckwheat, white clover, peas, and 

 beans can complete their life cycle from seed to seed under continuous arti- 

 ficial illumination. Presumably only species of the long-day or indeterminate 

 types will show such a reaction to continuous illumination and even some of 

 them are injured or killed when exposed to such conditions (Arthur, 1936). 



The results of the numerous experiments which have been performed on 

 photoperiodism indicate clearly that length of day influences not only the 

 quantity of photosynthate produced, but the use which the plant makes of the 

 compounds which it synthesizes. Present indications are that photoperiodic 

 effects in plants depend at least in part, and perhaps entirely, upon a hormo- 

 nal mechanism. The Russian investigator Cajlachjan (see Garner, 1937) 

 has shown that metabolic processes caused by changes in length of day which 

 lead to flowering occur in the leaf tissues, but are entirely distinct from car- 

 bohydrate synthesis. The influence of these processes is apparently trans- 

 ferred from the leaves to growing points by material substances of a hormonal 

 nature. The name florigen has been proposed by Cajlachjan for the postu- 

 lated "flower hormone." Results of Loehwing (1938 a) also point in this 

 direction. When the tops of soy beans of the short-day Ito San variety 

 were exposed to 9 hours daily illumination, and the base to 14 hours, flowers 

 formed only on the tops. When the base received a daily photoperiod of 

 9 hours and the tops of 14 hours, flowers developed only on the bases. How- 

 ever if the top was first defoliated and exposed to a 14 hour day-length, flowers 

 developed on it if the base was exposed to a 9 hour day-length and kept ex- 

 florated. Apparently under a short photoperiod specific substances which are 

 necessary for flowering were synthesized in the leaves, and, under the condi- 

 tions of this experiment, these hormone-like substances were translocated into 

 the tops of the plant. Similarly if the base was defoliated and exposed to 



