468 The American Naturalist. [May, 
5. Regular periodicity in the tuber continues after the perio- 
dicity*of the aérial stem is suspended. 
6. Connected with profound changes of condition in the 
_ aérial stem changes in the periodicity of the tuber may be noted. 
7. There is some connection between the periodic growth of 
the tuber and the periodic growth of the aérial stem. What this 
connection is does not appear. 
8. There is also, it is probable, an independent periodicity in 
the growth of the potato-tuber which is obscured and modified 
by the secondary induced periodicity, which is related to conditions 
of the aérial stem and its mode of growth. 
Passing now to conjectural explanations of the observed peri- 
odic growth of the potato-tuber, it may be affirmed that very little 
can be expected at this stage of the investigations. Whether like 
embryonic shoots of Hedera, with their heliotropic irritability, the 
potato-tuber retains, somehow, in hereditary fashion, its above- 
ground periodicity, and thus gives hint of the time when its pre- 
cursors were exposed to rhythmic alternation of light and dark- 
ness, is entirely an open question. On the other hand, it is equally 
uncertain whether the induced periodicity is due to one or many 
causes. Some lines of attack are indicated below, and it is hoped 
that they may be followed to their rational gonclusion. ; 
1. The rhythm of assimilation in the above-ground stem ud 
affect the growth of the below-ground tuber. The synthesis of 
carbohydrates is a diurnal affair. From these carbohydrates the 
substance of the tuber is formed. Thus the rhythm above might 
induce a rhythm below. 
2. The conversion of plastic into reserve materials is character- 
istic of an organ like the tuber. This conversion depends upo? 
the activity of certain ferments which are results of destructive — 
and constructive metabolic changes in the shoot area. Tien 
metabolic changes are consequent upon the respiration function, 
and this is a rhythmic function. sod 
3- The growth of the above-ground stem is strongly pent 
and demands, in any plant, the same kind of material which w 
be supplied to a growing tuber. This drain upon the piei 
rial in one direction might induce a corresponding dearth + 
