900 
Journal of Agricultural Research 
Vol. XXIII, No. « 
therefore, that the length of day is a factor in the so-called ‘‘law of lever' 
applied to underground stems. Thus, in effect, a change in day length 
from optimum to suboptimum for apogeotropic stem elongation may 
cause the apical bud to lose its dominance and lateral buds to become 
active. As stages in the response to departure from the optimum light 
exposure for increase in stature may be noted branching at the top, 
middle, or base of the aerial portion of the axis; various degrees of 
erectness in the resulting branches, from the upright to the pendent 
form; the prostrate or creeping form of stem elongation; and develop¬ 
ment only of the underground type of stem which, in turn, may change its 
direction of growth upward or downward with a change in the daylight 
period toward or away from the optimum for negative geotropism. 
ROOT GROWTH 
Preliminary observations indicate that the duration of the daily 
illumination period may exert a marked effect on the relative develop¬ 
ment of the root and tie aerial portions of the plant. For example, a 
cutting of Biloxi soybeans made no top growth at all through the winter 
months and the original leaves assumed a very dark color and generally 
unthrifty appearance. Apparently new buds were unable to develop. 
Upon examination of the underground portion of the plant in the spring 
it was seen that the soil contained a large mass of roots altogether out of 
proportion to the top of the plant, as judged by the usual summer growth. 
Other similar cases have been observed in which a light duration un¬ 
favorable to aerial development has caused extensive root growth. 
Growth of root and shoot, therefore, are not necessarily contemporaneous 
with respect to season, and arrested development of the exposed portion 
of the plant caused by suboptimal light duration need not be accompanied 
by checking of root growth. 
PUBESCENCE 
A feature of the experiments with Amaranthus hybridus L. described 
on page 873 worthy of note is the fact that under the shortened light 
exposures of 5, 10, 12, and 13 hours daily the leaves were glabrous and 
the veins were not prominent. The leaves of the control plants, exposed 
to the full-day length, on the other hand, were noticeably pubescent and 
the veins were decidedly more prominent. 
Specimens of wild lettuce (Lactuca spicata (Lam.) Hitchc.) were trans¬ 
planted to boxes from the field March 29 when in the form of small 
rosettes. Under a io-hour day the first blossoms opened August 24, 
and under the full summer length of day the first blossoms opened 
August 21. The average heights were 65 and 96 inches, respectively. 
Under the shortened light exposure the reduction in stem elongation was 
accompanied by a marked increase in the size and in coarseness in tex¬ 
ture of the leaves, as shown in Plate 10, B. There was, also, a striking 
increase in the pubescence and roughness of the leaves and stem under the 
short daylight exposure. 
Thus, in the case of the short-day plant Amaranthus pubescence was 
not favored by shortening the light period, but in the case of the long-day 
plant Lactuca the shortened daylight period was markedly effective in 
this respect. 
