Mar. *7, X933 
Further Studies in Photoperiodism 
889 
Similarly, since the general change from long to shorter days with the 
advent of late summer would favor vegetative activity, the growth and' 
development of flower buds would be correspondingly retarded and, 
finally, cold weather, also, would operate to slow down both types of 
activity. Thus, the tendency would be toward sparse fruiting and rapid 
vegetative development with advance from high latitudes toward the 
equator; the shortening days of late summer would tend to delay final 
flowering till the following spring, particularly in high latitudes. 
FORMATION OF BUEBS, TUBERS, AND THICKENED ROOTS 
Tuberization is one of the outstanding features of photoperiodism 
This fact is not so surprising, perhaps, in view of the significane of stem 
elongation as a fundamental factor in various responses of the plant to 
change in the daylight period. As is well known, tubers and bulbs 
represent extreme conditions of stem shortening accompanied by marked 
increase in diameter. Using the term tuberization in its broadest sense 
to include the formation of thickened roots as well as the more typical 
tubers, bulbs, and corms, the general feature of carbohydrate accumula¬ 
tion may be regarded as being common to all these storage organs. This 
accumulation obviously indicates an excess of carbohydrate over current 
consumption, but the important point to be emphasized in the present 
discussion is that the surplus of carbohydrate is not due to increased 
photosynthetic activity but rather inability on the part of the plant to 
utilize the carbohydrate, whether it be present in relatively large or small 
quantity. 
As to the locus of the tuber or other storage organ it appears that the 
presence of previously formed buds or the ability to organize such struc¬ 
tures is an important factor. Environmental conditions also play a part 
in determining the position of storage organs as well as in their forma¬ 
tion. The work of Vochting on this subject is of special importance. 
This investigator (22) showed that shoots of the potato (Solanum tubero¬ 
sum L.) set in the soil in such manner that the underground portion 
consists only of an intemodal section readily forms aerial tubers for the 
reason that the underground portion is incapable of developing adventi¬ 
tious buds. It was shown, also, that by suitably darkening the apex of 
the primary shoot on a branch the growing point is converted into a 
tuber and, consequently, it was concluded that darkeness favors tuber 
formation. 
In later experiments with tubers of the Marjolin variety of potato 
taken from storage after having sprouted (23) Vochting found that when 
germinated in darkness and with abundant moisture in soil and air 
roots and foliage shoots quickly developed but no new tubers were formed 
if the temperature was maintained at 25 0 to 27 0 C. With a temperature 
of 5 0 to 7 0 , on the other hand, no foliage shoots were formed, but instead 
numerous new tubers developed at the ends of old sprouts and new stolons 
and on the mother tuber. In other words, the growing points were 
tuberized. By transfer from the higher to the lower temperature the 
foliage shoots assumed horizontal direction of growth and formation of 
tubers began. Experiments with low partial pressures of oxygen indi¬ 
cated that decreased respiratory activity \s not responsible for the low- 
temperature effect. It i s important to observe, however, that germination 
in sand free from moisture gave no foliage shoots but only short roots 
and stolons bearing numerous tubers, regardless of whether the tern- 
