Mar. 17, 1933 
Further Studies in Photoperiodism 
901 
PHYSIOLOGICAL TRANSLOCATION 
As was brought out in the preceding discussion of apogeotropism and 
of branching, a day length sufficiently below the optimum for apogeo- 
tropic expression often results in the leaf-rosette type of development 
(or prostrate form of stem growth), with more or less complete inhibition 
of upright stem elongation. Again, where upright growth of the primary 
axis has already taken place a change to suboptimum in the light duration 
tends to break down the dominance of the apical bud over the lateral 
buds, thus causing development of either aerial or subterranean secondary 
stems or branches, depending on the species in question and on the 
extent of the change in light duration. A third important phase of 
apogeotropism now requires consideration. Under the influence of the 
apogeotropic stimulus the lower parts of the plant, including leaf, stem, 
tuber, and thickened root, may yield up their nutritive constituents 
to the upper plant parts and for the most part tend to pass into a state 
of senescence, often followed by death. Thus, under the influence of a 
favorable light exposure the plant in increasing its stature may use over 
and over again, within limits, elements of plant food which may be avail¬ 
able only in limited quantities. Conversely, under an unfavorable light 
duration the movement of plastic nutrient material is downward rather 
than upward, with resultant increased growth of the lower plant parts 
(branching, stooling, stolon formation, development of rootstocks, 
growth of roots, etc.) or, with a still less favorable light duration, forma¬ 
tion of underground storage and resting organs. With only a moderate 
departure from the optimal light duration for increase in stature the 
effect may be to initiate flowering. In this case translocation toward 
the apex is not interrupted unless the change in the illumination period 
is too great. In the latter event the movement of food materials is 
diverted downward and successful fruiting is prevented, as, for example, 
in poinsettia exposed to a io-hour day (see p. 876). 
In a recent investigation Schertz (21) concluded that mottling of the 
older leaves of Coleus blumei is due to a failing nitrogen supply. There 
can be no doubt of the fact that with only a limited supply of nitrogen 
available there is often a decided tendency for the lower leaves to yellow 
and wither, while the upper portion of the plant continues to grow. 
For example, it is well known by tobacco growers that too low a propor¬ 
tion of nitrogen to phosphorous in the soil will cause “firing” or dying 
of the lower leaves of the tobacco plant. It still remains, however, to 
explain why the lower, more mature leaves yield their nutrient materials 
to the upper portion of the plant when the available supply of these 
nutrients is limited. It seems reasonable to suppose that this phenom¬ 
enon is largely a manifestation of apogeotropism, and it appears that 
appropriate duration of the daily illumination period is a controlling 
external cause of this manifestation, whatever may be the internal 
mechanism through which it operates. 
Thus, when exposed to a long day Cosmos has attained a stature and 
size out of proportion to the small volume of soil supporting the root 
system in the container. Under these conditions, however, the lower 
leaves soon perish so that only a comparatively narrow zone of living 
foliage immediately beneath the growing part follows the upward growth 
of the stem. In similar manner the lower portions of the stem tend to 
become woody and retain only minimal quantities of the plant food 
elements needed by the upper growing portion. The conclusion that 
