908 
Journal of Agricultural Research 
Vol. XXin, No. IX 
shrubs, including Acer negundo L., Acer rubrum L., Cornus florida L., 
and Rhus glabra L., which were exposed to an 8-hour day, beginning 
early in January, successfully unfolded their leaves at about the same 
time as did the controls. Some of these, however, made but little sub¬ 
sequent growth even under a io-hour day, which proved to be too 
short for their requirements. It appears, therefore, that there are all 
degrees of dependence upon the direct action of the light period in the 
matter of emerging from dormancy. Much, doubtless, depends on the 
conditions under which the plant enters dormancy and the environmental 
conditions which prevail during the rest period. The whole subject of 
dormancy in relation to the light period will require further study before 
general conclusions may be drawn. 
SENESCENCE AND REJUVENESCENCE 
All experimental data thus far secured seem to support the conception 
advanced long ago that a common cause of general senescence, that is, 
senescence of the whole or greater part of the plant, is the intense forma¬ 
tion of independent or semi-independent reproductive structures. In 
annuals a length of day which favors intense flowering and fruiting has 
resulted in rapid senescence and death, and in herbaceous perennials 
this has been true with respect to both sexual reproduction and tuber 
formation. This relationship applies whether the optimal light period 
for reproduction corresponds to the longest summer days or the shorter 
days of spring and fall. It has been pointed out that Peking soybeans 
rapidly pass into senescence under the influence of a day length of-about 
13 hours, which approximates the optimal length of day for flowering 
and fruiting. Under a light period much in excess of, or much below, 
13 hours senescence is deferred. Convolvulus tricolor L. furnishes an 
example of how a light period too short to admit of flowering may 
indefinitely delay decline and death. Seedlings exposed to a io-hour 
day, beginning June 8, were unable to blossom, while growth continued 
for a period of more than eight months. In this instance the plant 
attains an abnormally large size by continued exposure to a light period 
too short to admit of flowering. 
Localized senescence is a very common feature of photoperiodism and, 
of course, is frequently accompanied by regeneration phenomena. When 
Cosmos, for example, is exposed to a long day, continued elongation of 
the axis is accompanied by progressive loss of lower leaves. As the 
light period becomes less favorable for apogeotropic response the ten¬ 
dency is in the reverse direction. In various species there may be simple 
abscission and leaf fall, accompanied by formation of resting buds; 
branching, as a result of loss of dominance on the part of the apical bud, 
followed by decline of the latter; progressive decline of the upper portions 
of the primary axis; a general transfer of activity from above to the 
ground level or even to subterranean parts. Examples of all these 
stages have been presented in the preceding pages. 
Since certain light periods may defer the appearance of senescence, 
the question arises as to whether exposure to these light periods may 
effect rejuvenation in cases of advanced decline and impending death. 
Examples of this type of rejuvenescence were given in the previous 
paper (7, p. 567) in the cases of soybeans, Ambrosia artemisiifolia L., and 
wild aster. Btdens frondosa L. also furnishes an excellent example. 
