682 BIOLOGICAL EFFECTS OF RADIATION 



Three years later the same authors pubUshed a second paper (23) in 

 which data are presented tending to confirm the results previously 

 obtained and to show that the length-of-day factor affects various features 

 of growth and development other than sexual reproduction. Observa- 

 tions were made on effects of day length in hastening or retarding flower- 

 ing and fruiting in many additional species and varieties, and it was 

 shown that under suitable conditions the formative action of the light 

 period may be localized in the plant. Particular attention was given 

 to the formation of bulbs, tubers, and thickened roots and the more or 

 less antithetical process of stem elongation, as affected by the daily light 

 period. Other influences of the light period studied include those on 

 character and extent of branching, root growth, formation of pigment, 

 pubescence, abscission and leaf fall, dormancy, senescence, and rejuven- 

 escence. It is concluded that the duration of the daily illumination 

 not only influences the quantity of photosynthetic material formed but 

 also may determine the use which the plant can make of this material. 

 It appears that for each species there is an optimal light period for maxi- 

 mum upward elongation of the stem or increase in height. In certain 

 typical cases this optimal period is furnished by the long summer days 

 of high latitudes, and progressive shortening of the day length may 

 initiate a series of responses including flowering and fruiting, branching 

 tuberization, and entrance upon dormancy. 



Shortening the day length by a certain decrement below the optimum 

 for stem elongation may induce intense reproductive activity and in 

 annuals this is commonly accompanied by rapid senescence and death. 

 Reduction in the light period to a point intermediate between the opti- 

 mum for stem elongation and the optimum for flowering favors a com- 

 bination of the two types of activity, as manifested in the ever-blooming 

 condition. Reduction of the light period below the optimum for stem 

 growth, and usually below the optimum for flowering, may induce 

 intense tuVjerization. Here, again, with an intermediate light period 

 sexual reproduction and formation of tubers may proceed simultaneously. 

 Change in the light period from optimum to suboptimum for stem elonga- 

 tion may cause the apex to lose its dominance, thus promoting branching. 

 Depending on the extent of the change in the light period, increased 

 branching may occur at the upper, middle, or lower aerial portions of the 

 axis; there may be various degrees of ercctness of the branches; or there 

 may be increased development of underground stems and a change 

 downward in direction of their growth. 



Further details of the results reported in these two papers and their 

 interpretation are given in connection with the following outline of the 

 development and the present status of principal phases of the day-length 

 effect. Photoperiodism in its various manifestations and its relation- 

 ships with other environmental influences has been the subject of exten- 



