1905] McCCALLUM—REGENERATION IN PLANTS 245 
Driescu (1), who has worked on animals more than on plants, 
recognizes in correlation the dominant factor in regeneration, and 
daims that the absence (Nichtmehrvorhandensein) of the one part 
is the cause of the development of the other. The plant, according 
to him, is not influenced in regeneration by external factors, but 
being sensitive to something lacking endeavors to replace it. Some- 
what analogous to this is NoLu’s idea (14) of a body-forming stimulus 
(Kérperjormreizen), by which he implies that there is an innate impulse 
in the organism toward a definite form, and when a part is removed 
the resulting disturbance (Formstérung) acts as a stimulus to the 
reconstruction of the whole. 
Such hypotheses as these are at present as incapable of demon- 
stration as they are of refutation, and can only serve a useful purpose 
if they form the starting point for experimentation. Unfortunately 
they can scarcely be said to do that. Quite different from these is 
the view held by KiEBs (9, p. 109), who believes that the removal 
only serves to bring about those conditions, such as accumulation of 
moisture, changes of a nutritive nature, etc., which would under all 
other circumstances cause a similar development. To take a specific 
case, the normal absence of roots on the stems of Salix, KLEBS says, 
is due to the retarding influence of light, of dry air, and to the fact 
that the water is being used by the leaves and young parts. When 
these conditions are supplanted by those of moist air and abundance 
of water, the roots develop quite independently of any removal or 
wounding. The experimental evidence that follows shows, however, 
that the problem is much more complicated than this. 
The experiments described have shown the dependent relation 
that exists between the growth of the apex and the non-development 
of the buds below. On Phaseolus the basal primordia do not 
develop so long as there are buds above them developing. Indeed, 
only one bud is sufficient for this; for if the upper part be cut away 
and all the buds but one be removed, the basal buds do not start. 
Experiment 40.—The stems were cut off at the second internode, 
and the buds from one side of the base of this were removed; and in 
Some cases not only the bud but the leaf and one-half the diameter 
of the epicotyl for its entire length was sliced away. The remaining 
ay eoroUsly, but neither of the buds at the base developed. 
y time this bud was cut away those at the base promptly started. 
