936 STUDIES IN PLANT REGENERATION 
in which the regeneration of these parts has been recorded, опе: 
or both structures are formed only at the base. This has always 
seemed to indicate a serious flaw in Goebel’s hypothesis. 
The teleological explanation of the early and general appear- 
ance of roots on nearly all kinds of plant-cuttings is a tempting 
one. It might seem as if roots formed easily because they are 
immediately necessary to the life of the part. But though the ad- 
vantage cannot be denied, it cannot be considered as an explana- 
tion of their appearance. In nature, the opportunities for regenera- 
tion do not occur in a sufficient number of individuals to allow the 
faculty to become established by natural selection; and if only 
roots are formed, as so often happens, even the advantage is only 
transitory, 
Other interpretations which readily suggest themselves also 
fail to hold on closer scrutiny. It might be thought that the 
simpler structure of the roots as compared with the shoots would 
account for the facility with which they are produced ; but when a 
part has had at its disposal during many months building-material 
enough to construct an elaborately branched root system out of 
all proportion to the needs of the cutting, the question may well 
be asked why some of it was not at once expended in the genera- 
tion of a shoot, which would thus have reproduced the plant. Nor 
can external conditions here be considered causal, since it has been 
shown that these may be varied radically with only arresting or 
slightly modifying differences in the result. 
The only recourse, therefore, seems to be to go back to the old 
formative stuffs idea in perhaps a somewhat altered form. If a part 
fails to produce a certain organ when food is plentiful, and even 
massed near a cut, when all relations between the piece and other 
growing regions are severed, when all hindrances to growth are 
removed, and when external conditions are at an optimum for such 
formation, it would seem to indicate that what is lacking is a very 
definite substance which it is not always in the power of the cutting 
to supply. In what guise is this substance to be regarded? То а5- 
sume with Sachs that the Sap contains two different materials, one 
heavy and root-forming which moves downward, and one lighter 
and shoot-forming which moves upward, owing to the action of 
gravity, blocks, as Vóchting * has justly intimated, the way to fur- 
"Bot: Zelt: Ga: geek 1906 22-222 
