■Mar. 17, 192$ Further Studies in Photoperiodism 
large number of species. While in a number of species there is a decided 
tendency toward eventual recovery from the condition of etiolation 
caused by a light exposure of very short duration, the reversed condition 
of increased depth of green color induced by a somewhat longer light 
exposure appears to be permanent so long as the proper light conditions 
prevail. This dark green color is associated with a condition of relative 
inactivity in the green portion of the plant so far as appearance goes, 
for typically there is little or no growth above ground. There is usually 
intense activity underground in this case in the formation and develop¬ 
ment of underground stems, tubers, and roots. 
It is hardly necessary to discuss at length the fact that the changes 
in color of foliage leaves which are regularly associated with autumn 
leaf fall may follow in a natural manner as a sequel to flowering, fruiting, 
and other phenomena leading up to senescence, dormancy, or death of 
the plant when these processes are developed through artificial control 
of the light period. It will suffice, perhaps, to give a single, very inter¬ 
esting case illustrating the fact that color changes affected by the light 
duration are not confined to the chlorophyll pigments. A specimen of 
poinsettia forced into flowering by exposure to a io-hour day, which 
was accompanied by the usual rich coloration of the bracts of the 
inflorescence, as described on page 876, was transferred September 2 to 
the greenhouse electrically illuminated from sunset till midnight. Under 
the influence of the lengthened illumination period a tendency toward 
replacement of the red color of the bracts by chlorophyll green soon 
became evident. The bracts persisted for many weeks, the younger 
ones assuming a clear green color. Finally, in December vegetative 
growth was resumed at the extreme tip of the inflorescence. The first 
growth of the new shoot was of the weak, pendent type, and the stem 
was red in color instead of the normal green. Eventually, however, the 
stem assumed upright growth and developed the usual green color. 
These color relations are well shown in Plate 1. 
ABSCISSION AND LEAF FALL 
In the case of such annuals as regularly shed their leaves before dying 
abundant evidence already has been given of abscission and leaf fall as 
phenomena following flowering and fruiting induced by suitable regula¬ 
tion of the length of day. Soybeans furnish a good example. Under 
the proper length of day flowering and fruiting are promptly followed 
by yellowing of the leaves, which take on a rich golden yeflow color. 
Abscission is soon completed and, as a result, the stalks are practically 
bare of leaves when final maturity is reached. These effects are clearly 
shown in the previous article (7, PL 66 and 69). 
Specimens of smooth sumac {Rhus glabra L.) were transplanted to 
wooden boxes in April and allowed to grow under outdoor conditions 
during the summer. On September 8 one lot was transferred to the 
greenhouse electrically illuminated from sunset till midnight while the 
second lot was placed in the greenhouse receiving natural light only. 
Under the latter conditions the leaves soon colored, although leaf fall 
was slightly delayed and the colors were not as bright as normal for 
the species. Under the lengthened illumination period there was no 
autumn coloration or falling of leaves. With the added electric illu¬ 
mination of very low intensity no additional top growth took place, but 
the leaves assumed an abnormally dark green color which was retained 
