6o4 Journal of Agricultural Research voi. xviii, no. h 



(3) In the absence of the favorable length of day for bringing into ex- 

 pression the reproductive processes in certain species, vegetative develop- 

 ment may continue more or less indefinitely, thus leading to the 

 phenomenon of gigantism. On the other hand, under the influence of a 

 suitable length of day, precocious flowering and fruiting may be induced. 

 Thus, certain varieties or species may act as early- or late maturing, de- 

 pending simply on the length of day to which they happen to be exposed. 



(4) Several species, when exposed to a length of day distinctly favor- 

 able to both growth and sexual reproduction, have shown a tendency to 

 assume the " ever- blooming " or "ever-bearing" type of development — 

 that is, the two processes of growth and reproduction have tended to 

 proceed hand in hand for an indefinite period. 



(5) The relationships existing between annuals, biennials, and peren- 

 nials, as such, are dependent in large measure on responses to the pre- 

 vailing seasonal range in length of day. In many species the annual cycle 

 of events is governed primarily by the seasonal change in length of day, 

 and the retarding or more or less injurious and destructive effects of 

 winter temperatures are largely incidental rather than fundamental. 

 Hence, by artificial regulation of the length of the daily exposure to 

 light it has been found that in certain species the normal yearly cycle 

 of the plant's activities can be greatly shortened in point of time, or, on 

 the other hand, it may be lengthened almost indefinitely. In certain 

 cases, annuals may complete two cycles of alternate vegetative and 

 reproductive activity in a single season under the influence of a suitable 

 length of the daily exposure to light. Similarly, under certain light 

 exposures some annuals behave like nonflowering perennials. 



(6) In all species thus far studied the rate of growth is directly pro- 

 portional to the length of the daily exposure to light. 



(7) Although the length of the daily exposure to light may exert a 

 controlling influence on the attainment of the reproductive stage, experi- 

 ments reported in this paper indicate that light intensity, within the 

 range from full normal sunlight to a third or a fourth of the normal, and 

 even much less, is not a factor of importance. It follows that the total 

 quantity of solar radiation received by the plant daily during the summer 

 season, within the range above indicated, is of little importance directly 

 so far as concerns the attainment of the flowering stage. 



(8) In extensive tests with soybeans, variations in the water supply 

 ranging from optimum to a condition of drought sufficient to induce 

 temporary wilting daily and to cause severe stunting of the plants were 

 entirely without effect on the date of flowering, although in some cases 

 drought seemed to hasten somewhat the final maturation of the seed. 

 Similarly, differences in light intensity, in combination with differences 

 in water supply, failed to change the date of flowering in soybeans. 



(9) The seasonal range in the length of the day is an important factor 

 in the natural distribution of plants. 



