The regulatory formation of mechanical tissue.’ 
FREDERICK C. NEWCOMBE. 
The prevalent notion of the influences which affect the pro- 
gress on growth is doubtless that which one finds in the 
manuals on plant physiology most in use at the present day. 
In these manuals growth is set forth as governed largely by 
mechanical forces; by increase and decrease of pressure, and 
this pressure not acting as a stimulus but as a mechanical 
force. In Sachs’ ‘‘Lehrbuch” of 1874 and again in his 
‘‘Vorlesungen” of 1882 is a detailed elaboration of his me- 
chanical theory; Vines’ ‘‘Physiology of Plants” (1886) follows 
Sachs closely; while Pfeffer’s ‘‘Physiologie” (1881) shows a 
little breaking away from the theory of Sachs, in that he 
gives contact as a stimulus to the formation of*tissue, a 
phenomenon therefore of irritability. 
It may not be devoid of interest to review briefly the steps 
by which this mechanical theory of growth gained its hold in 
the minds of botanists. 
As the result of his well-known experiments, described in 
1803, Knight? expressed the belief that the cortex of trees 
by its pressure exerted a restraining influence on growth in 
diameter, and that any means which reduced this pressure, 
such as the swaying by the wind, would allow a greater flow 
of nutritive fluid to the place and hence promote growth 
In 1859, Hofmeister® established the fact of interacting 
tissue tensions in a longitudal direction in growing plant 
organs. 
Kraus,* in 1867, followed up the work of Hofmeister and 
discovered transverse tissue tension. Among his conclusions 
is this: that the curvatures due to geotropism, heliotropism, 
shaking by storms, and so forth, are accompanied by an ex- 
cessive growth on the convex side, this excentricity of growth 
1Read before Section G of the A. A. A. S. at the Brooklyn meeting, August 
I 
*Knight, Philos, Trans. Roy. Soc. Lond. —: 280-3. 1803. 
® Hofmeister, Ueber die Beugung saftreicher Pflanzentheile durch Erschiit- 
terung. Ber. d. k. sachs. Gesell. d. Wiss. —: 194. 1859. ; 
Bean ny Die Gewebespannung des Stammes und ihre Folgen. Bot. Zeit. 25: 
105. 1867. 
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