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1900] MECHANISM OF ROOT CURVATURE 9: 
Some experiments on tissue tensions showed that when curved 
stems of Pisum were split into two equal parts both parts curved 
outward, that is, the concave side curved more and the convex 
side straightened. If the stem is split in parallel planes into 
three parts the concave cortex shortens and the convex cortex 
and pith remain about the same length. Kohl implies, though 
he does not directly say, that the tissue tensions indicated by the 
result obtained were due to the stimulus. In a footnote to his 
preface Kohl says he regards the stimulation curvatures as merely 
phenomena of tissue tension. Yet in his book (33) he devotes 
less than three pages to tissue tensions, and in his discussion he 
is only half right because he ignores the tissue tensions existing 
previous to the application of the stimulus and independent of it. 
Pfeffer (56) reports some very searching experiments on 
stimulated grass nodes, and incidentally gives his views on the 
mechanism of curvature. The anatomy of grass nodes is very 
different from that of curving roots and most stems. The curv- 
ing takes place only in the cushion of the leaf sheath at the node. 
The middle tissue of the cushion consists of a hollow cylinder 
of vascular bundles accompanied by a very large amount of 
collenchyma. This cylinder is bounded both internally and 
externally by thin-walled parenchyma. The tissue tensions are 
such that the parenchyma, freed from the bundles, lengthens 30- 
40 per cent., while the bundles contract less than I per cent. 
In the process of curving, when the grass stems are placed in a 
horizontal position, the growing collenchyma on the lower side 
frees the parenchyma on that side from its positive tension, and 
may even put it under a negative tension to such a degree that 
it is frequently broken in transverse clefts to the depth of the 
bundle. Thus the passive stretching of the convex side which 
Kohl supposed was produced by the contraction of the concave 
side, proves to be due to the reversed tensions between two tissues, 
both of which are in the convex side (56, p. 407). This applies 
to grass nodes only. Pfeffer’s results prove conclusively that 
the contraction theory cannot possibly account for the curve 
in grass nodes, and Wortmann’s theory is even less adequate. 
