1897 ] THE CURVATURE OF ROOTS 309 
lar spaces, which may receive any amount of water liable to be 
freed from the highly permeable motile cells. In other tendrils, 
in which intercellular spaces are not to be found in the cortex, 
the connection with the conducting tissue is direct and evident. 
I have recently called attention to the readiness with which large 
drops of water exude from the cut surfaces of active tendrils, 
which indicates the facility with which great quantities may be 
conducted to or from any point in the organ.’ 
In the motor zone of roots, however, no such intercellular 
spaces are to be found, and vascular tissues are not fully formed 
as yet; hence sudden or great variations in the water content of 
any of the cells in cross section is not possible. As necessary 
concomitants of these conditions, the movements brought about 
in roots follow the stimulus only after a much longer latent 
period, since movement can only be accomplished by alterations 
in the mass of the entire tissue together, while in tendrils the 
individual cells are capable of undergoing changes in form and 
size by giving off or taking up water from the intercellular spaces 
bounded by their outer walls. 
II. HISTORICAL AND GENERAL. 
The curvatures of roots have been regarded as identical with 
the movements of other organs, and the development of the 
present knowledge of the subject is to be found in the older 
literature under the title of curvatures. It will be conducive to 
clearness to recall the more important theses which have received 
Support at various times, so far as the causes of curvatures are 
concerned. In the special paragraphs dealing with the curva- 
tures of roots, the history of the researches bearing upon the 
action of these organs will be given. 
Perhaps the first actual observation of facts concerned in the 
mechanism of curvatures was made by Hofmeister (9, p. 88). 
He found that the extensibility of the epidermal membranes of 
the convex side of an onion stalk was increased after geotropic 
*On traumatropic curvatures of tendrils: A paper read before the Indiana Acad- 
ees of Science, Indianapolis, Indiana, December 1896. 
