Vol. XXIII, No. ii 
918 Journal of Agricultural Research 
period is given somewhat detailed consideration in its apparent relation 
to flowering and fruiting and the other responses mentioned. 
It appears that the duration of the daily illumination period not only 
influences the quantity of photosynthetic material formed but also may 
determine the use which the plant can make of this material. The 
evidence available at this time indicates that for each species there is 
an optimal light period for maximum upward elongation of the stem or 
increase in height. For some species this optimal light period is fur¬ 
nished by the longest days of summer in the temperate zone while for 
other species the intermediate length of day of spring and fall is optimal. 
In the latter group of plants tlie excessive length of day of midsummer 
probably initiates flowering, and in the case of some woody perennials 
the formation of resting vegetative buds. Certain species, moreover, are 
caused to die back, form bulbs, and enter into a midsu mm er rest period. 
In the first-named group of species progressive shortening of the 
daily light period initiates a series of responses, including flowering and 
fruiting, tuberization, character and extent of branching, dormancy, 
all of which are prominent features in annual periodicity. Reducing 
the light period below the optimal for stem elongation by a certain 
definite decrement tends to divert the plant's activities more or less 
quantitatively toward sexual reproduction. There seems to be an opti¬ 
mal light period, therefore, for flowering and fruiting. Exposure to 
this optimal light period commonly induces rapid senescence and death, 
as typified in annuals, probably as a result of the intense flowering 
and fruiting. With day lengths ranging between the optimal for growth 
and the optimal for sexual reproduction the tendency toward division 
of the plant's energies between these two types of activity is manifested 
in the everblooming or everbearing condition, shifting of the quantitative 
relationship between relative vegetative and reproductive activity, 
change in the size of the individual fruit or seed, delayed and sparse 
flowering and fruiting, cleistogamy, and other abnormalities. Moreover, 
in the natural seasonal change in length of day there is considerable 
difference between the effects of the change away from the vegetative 
optimum toward the flowering and fruiting optimum and those result¬ 
ing from the change in the opposite direction. Data of interest have 
been presented, also, on the time required for imparting the stimulus 
or condition for flowering by appropriate change in the light period. 
Further reduction of the light period by a sufficient decrement below 
the optimal for sexual reproduction tends to induce intense tuberiza¬ 
tion, a feature marking the final stages in reduction of stem elongation. 
There is a more or less clearly defined optimum for tuberization. Here, 
again, with a light period ranging between the optimal for tuberization 
and the optimal for flowering there is a tendency toward division of the 
energies of the plant between these two features of development. 
Under these conditions sexual reproduction and tuberization may pro¬ 
ceed simultaneously, a division of activity which is commonly seen in 
herbaceous perennials. Tuberization as a result of decrease in the daily 
light period involving deposition of carbohydrate in relatively con¬ 
densed or dehydrated forms indicates marked loss of power to utilize 
the product of photosynthesis in elongating the stem or in developing 
flower and fruit. It is most typically associated with the stemless or 
rosette form of foliage development, representing, perhaps, maximum 
divergence between income and effective utilization of the photosyn- 
t hetic product for promotion of growth. 
