12 SELBY: STUDIES IN ETIOLATION 
plants showed a tendency to start into activity during the growth 
of the main stem. The leaves were dropped from the etiolated 
stems, so that when the examination was made none were found 
below the tenth or twelfth internodes from the base ; although in 
a few instances bract-like organs formed on lower internodes were 
retained fora long time. Normal stems of the same age pro- 
duced but 7 to g internodes with an average length of 3-4 cm. 
It would appear from these facts that the abnormal length of the 
etiolated stems of this plant is due both to the exaggerated 
growth of the internodes and to the actual multiplication of the 
internodes. 
The anatomical features present some interesting comparisons 
which may be studied by reference to the figures. In the etio- 
lated stems (plate 5, figure 6) the cambium included four or five 
layers, while the woody cylinder and the bast-cells were less 
strongly developed than in the normal, as might have been ex- 
pected. Inthe middle of the normal stem it was noted that the 
bast-sheath was more nearly continuous than at the base; a like 
condition, though less strongly marked, was also observed in the 
middle of the etiolated stem. In one instance an etiolated stem 
had become nearly prostrate by its own weight, and the apical 
portion had responded by an apogeotropic curvature which brought 
the tip into an erect position. Little difference could be detected 
in the structure of the central cylinder of these stems with re- 
spect to the convex sides, but the bast-fibers, scarcely differentiated 
on the convex side, were developed as well-marked strands com- 
posed of colums of two or three dozen cells in cross-section upon 
the opposite or concave side. This seems to show that while 
stretching tension might induce the development and differentia- 
tion of this tissue, the absence of stress, or positive compression 
resulted in the non-differentiation. Similar effects have been noted 
in special investigations, by various workers, dealing with the effect 
of compression and tension upon the mechanical tissues. Deter- 
minations of water were made in the normal and etiolated stems 
of this plant with the following results : 
DRY SUBSTANCE IN PLANTS OF Asclepias incarnata 
ormal plant Etiolated plant per cent. 
cent. Il Average. 
Water 86.58 91.83 91.57 91.70 
Dry substance..... £3.42 8.17 8.43 8.30 
