1895. ] Regulatory Formation of Mechanical Tissue. 447 
stakes, and secondly, by the results which I have obtained 
by enclosing internodes of stems in plaster casts. 
A general result in all my experiments, performed on scores 
of plants, embracing twenty-five species and a dozen genera, 
is the lack of the development of the mechanical tissue. The 
casts employed were from 3™ to 5™ in length, and thus a seg- 
ment of the stem was freed from the most of the strain to 
which it would normally be subjected. There could be no 
lateral swaying, nor could the confined segment feel the full 
weight of the stem above. It is, of course, true that with an 
envelope of plaster, growth must soon be brought to a stand- 
still by mechanical means, and therefore that less than the 
normal amount of supporting tissue could be formed. But 
that the lack of formation of mechanical tissue within the en- 
appear from the two following reasons: In the first place, 
the young cells of the pith, of the collenchyma in the cortex, 
of the hard bast, and of the xylem that were formed before 
the casts were applied, did not, well within the casts, reach 
their normal thickness of wall. In the second place, corre- 
sponding tissues within but near the limits of the casts be- 
came thicker-walled than normally. Thus we have, at two 
places within the same cast, tissues in the one case, where 
there is little or no external stress, remaining abnormally 
thin-walled, but in the other case, where there is great stress, 
becoming abnormally thick-walled. This abnormal increase 
in the thickness of membranes in the region of the limits of 
cast is worthy of more than passing notice. It must be at 
the limits of the plaster envelopes that these plants felt the 
greatest strains from lateral swaying by the wind and from 
supporting the stem, as the breaking of several of them by 
the wind demonstrated. The thickening of membranes was 
always greatest just at the surface of the casts, and from that 
level it decreased both upward and downward, extending into 
the cast for a distance of a centimeter. The contrast was 
very striking; within the distance of a centimeter one could 
pass from a cross-section composed wholly of thin walls to 
one composed mostly of unusually thick-walled elements. All 
kinds of tissue took part in this great development, but espe- 
cially the pith and cortex, since the production of new cells 
rom the cambium was mostly prevented by the mechanical 
conditions. I see no way of explaining the results of these 
