72 



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 for a long time. Normal stems of the same age pro- 

 duced but 7 to 9 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. In the 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 



[Normal plant 

 per cent. 



Water 86.58 



Dry substance *3-42 



> OF Asclepias incarnata 



Etiolated plant per cent. 

 I II Average 



91-83 

 8.17 



91-57 

 8-43 



91.70 

 8.30 



