50 BOTANICAL GAZETTE . | JANUARY 
The root reacts to the stimulus as a whole, and is not affected 
merely on one side. The changes in the cells of the two oppos- 
ite sides, which become respectively concave and convex, are 
opposite in their effects, and these changes are coordinated so as 
to bring about the curvature with a fairly economical expendi- 
ture of energy, so far as this is indicated by the tissue tensions. 
It is remarkable that a stimulus may be transmitted in one side 
of the cortex past a deep gash in the opposite side, and yet 
above the gash produce opposite effects on the opposite sides of 
the root. The mechanism of the curvature is a more compli- 
cated problem than some writers on the subject have imagined. 
7. In the traumatropic curvature of Vicia faba roots, the zone 
of maximum curvature does not coincide with the zone of maxi- 
mum growth, but is nearer the tip. In four to eight hours after — 
the beginning of the curve, it is mostly in the first half-centi- 
meter from the tip, while the greater growth during the same 
period is in the second half-centimeter from the tip. Within — 
twenty-four hours, however, the zone of maximum growth meets — 
and may even pass the zone of maximum curvature. 
8. The proportions of cells in the curving part of Vicia roots ; 
are such that Kohl’s theory of the curvature cannot possibly be 4 
true for them. 
9. The cells of the convex half of curved roots contain 4 : 
larger quantity of water than those of the concave half. 
Theoretical considerations. 
After examining carefully the experimental evidence we are 
forced to the conclusion that no theory yet advanced is adequate — 
for the explanation of all curvatures due to stimulation. Sachs’ ; 
earlier view that there is merely a difference in the rate of growth 
on the opposite sides of the curving organ does not explain 
satisfactorily the contraction often observed on the concave side; 7 
and even if it did explain this contraction it would not be a com- — 
plete explanation of the curvature, because we do not yet know 
what growth really is; nor do we know the immediate source of — 
the energy of growth. The evidence is against de Vries’ view : 
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