120 
J. C. Th. Uphof. 
To explain the hygrochastic movement of the stem it is 
necessary to understand the anatomical structure. Leclerc du 
Sablon (14) states it is due to a difference in thickness in the cell 
walls on the two sides of the stem; the upper side, he says, is 
thick-walled, the lower thin-walled. Wojinorvi<5 (29) is of the same 
opinion : “ Die organisch Obere, bei der Zusammenrollung der 
Pflanze einwarts gekrtimmte unddaher concav werdende Seite wird 
aus Zellen mit starken verdickten Wanden gebildet als die organisch 
Untere, bei der Zusammenrollung aussere, also convex werdende 
Seite.” He states further that the thick-walled cells absorb more 
water than the thin-walled ones and the former lose more water 
during drought than the latter, causing the plant to curl up and 
to have the appearance of a cluster-ball. 
1 found after careful examination that the tissues are neither 
thicker-walled on the upper nor thinner-walled on the lower side of 
the stem, but precisely the reverse and also that the thin-walled 
