902 
Journal of Agricultural Research 
Voi xxin. No. n 
under the long-day influence there is active upward movement of 
nutrients from the lower portion of the plant seems to be supported by 
the fact that cutting back the stem quickly results in a swelling of the 
upper residual node, followed by rapid growth of branches (Pl. 17, A), 
an effect which is not observable under short-day conditions. Poinsettia 
shows even more markedly than Cosmos this tendency to continue 
upward elongation of the stem under the long-day stimulus in spite of 
a limited supply of nutrients from the soil, and, in doing so, to constantly 
shed the lower leaves as stem growth proceeds (PI. 19, B). 
For successful fruiting, after flowering has been initiated, it is essential, 
of course, that the reproductive structure receive from other plant parts 
an adequate supply of food materials. It is apparent that ordinarily 
such a supply of nutrients is assured under a day length approximating 
the optimum for initiating flowering. Under a length of day veering 
toward the optimum for apogeotropic response the direction of the food 
stream is not changed but the tendency is for diversion of the arriving 
food material to the development of vegetative structures, thus causing 
shedding of the blossoms or young fruit. A change in length of day 
in the opposite direction, however, if sufficiently marked will tend to 
direct the food stream downward, thus favoring the development of 
lower branches, leaves, bulbs, tubers, and roots. In some cases there 
appears to be an intermediate condition in which the food stream still 
moves to the upper parts (or is arrested before passing downward), 
but the light relations will permit neither vegetative nor floral buds to 
grow. Under these conditions aerial tubers may result, or in perennials 
the food material is simply deposited in the stem in the region of resting 
buds. 
PIGMENT FORMATION 
It is well known that continuous exposure to darkness for considerable 
periods of time ordinarily leads to etiolation. According to Bonnier (j) 
continuous exposure to electric illumination of an intensity much lower 
than that of sunlight causes a much greater development of chloroplasts 
and formation of chlorophyll than under normal conditions. In view 
of these facts the effects of differences in the length of the daily illumina¬ 
tion period are striking. Exposure to a very short day length com¬ 
monly leads to well-defined etiolation, but in some cases there is, after 
a time, more or less complete recovery, the tendency being toward a 
return to the normal green color. Under a 5-hour day turnip ( Brassica 
rapa U.) became extremely etiolated and died after attaining a height of 
only a few inches. On the other hand, peanut (Arachis hypogoea L.) 
under similar conditions showed only slight evidence of etiolation at 
first and, soon becoming noticeably greener, made comparatively good 
growth throughout the summer. Sweet potato (Ipomoea batatas L.) 
became severely etiolated but later partially recovered and made fair 
growth. Soybeans behaved much the same as sweet potato. Irish 
potato showed severe etiolation and made relatively poor growth. 
Aster linariifolius L. retained its green color throughout. Under a 
io-hour day Dioscorea batatas L- became markedly etiolated but sub¬ 
sequently almost completely recovered its normal green color. 
In striking contrast with these etiolation effects, exposure to a length 
of day in excess of that producing etiolation but still below the optimum 
for flowering usually causes the leaves of the plant to assume a shade of 
green much darker than the normal. This effect has been noted in a 
