244 BOTANICAL GAZETTE [ocroBER 
part monopolizes the nutritive material to such an extent that the 
other parts concerned cannot obtain sufficient to enable growth to 
go on; or (2) he applies Sacu’s .Stoff-jorm hypothesis. According 
to this (1'7), there are formed in the plant small quantities of different 
substances, presumably of enzyme-like nature, each one having the 
capacity to incite the formation of a definite structure. These sub- 
stances are supposed to move in definite directions, and where they 
accumulate in sufficient quantities start the development of the par- 
ticular structure they are concerned with. In GoOEBEL’s opinion 
(4, p. 204) the influence of external conditions are of little account 
in regeneration, the important cause being “the direction in which 
the constructive material moves.’”? GOEBEL says (3, p. 42) “the 
vegetative points act as centers of attraction for the plastic material, 
their influence being stronger or weaker according to their position.” 
In Bryophyllum, for example, the apex of the shoot is the strongest, 
then the lateral buds, and last of all the vegetative points on the 
leaves; so that the apex is able to draw to itself the greater part of 
the “constructive material;” but if this apex is removed, the lateral 
buds will be able to “attract” this substance; and in the absence 
of these lateral buds, the growing points on the leaf are able to appro- 
priate it. In Begonia no growing points are present on the leaf, 
but when it is removed GoEBEL says bud-forming material accu- 
mulates at the base and induces the formation of buds there. If this 
material, formed in the leaves, moves toward the base of the leaf 
and passes out because it is “attracted” by the growing points on 
the stem, just why it should continue to flow in that direction and 
accumulate at the base, when all connection with these “centers of 
attraction” is broken, is one of the unexplained difficulties that beset 
-this hypothesis on every hand. Morcan (12) has strongly objected 
to this theory, but his evidence against it does not seem to me to be 
necessarily fatal. GorBEL in a recent paper (5) is inclined to lay 
less stress on it than formerly, asserting that the non-development 
of the buds on the leaves is due to a checking influence exerted u 
the buds of the shoot; “but,” he adds, “whether we are here dealing 
with a stimulus transmitted along the conducting system, OT whether 
the building material (Baustoffe) flowing in the conducting chanm o 
is attracted more strongly by the shoot vegetative points than by 
those on the leaves remains uncertain.” 
