1897 ] THE CURVATURE OF ROOTS 313 
far as the mechanism of curvature is concerned, he concludes 
that it is due to the exaggerated growth of the convex side. 
Later researches by various investigators were turned toward 
other special phases of the subject and will be treated under the 
proper heads. 
It has followed, as a result of the various investigations 
named above, that the immediate cause of curvature of roots 
and similar parts must be looked for in the cell wall, rather than 
in the ectoplasm. In certain tendrils, on the other hand, the 
immediate cause of the curvature is the alteration in the motility 
and permeability of the ectoplasm. It is of course true that the 
changes in the cell wall in roots must be induced by the ecto- 
plasm. 
It has appeared to the writer that the more important facts 
concerning curvature might be obtained by an actual examina- 
tion of the changes of cell contours and wall characters in the 
motor zones of the curving organs, in the manner which has 
yielded such decided results in the study of tendrils, and which 
has been applied to some extent by Kohl in unicellular organs, 
and especially by Noll in his study of stems. The result may apply 
to all isodiametric organs, but the writer does not wish to make 
such strict and inclusive application, since it is conceivable that 
the disposition of the mechanical factors, as well as the devel- 
opment of the various forms of irritability, would necessitate in 
many cases a somewhat different method of procedure. 
IV. DEVELOPMENT OF IRRITABILITY IN ROOTS AND SHOOTS.” 
The emergence of the plant from an aquatic to a terrestrial 
habitat, in connection with the loss of motility in an extremely 
early stage of its development, was marked by several radical 
changes in its physiological organization, due in greater part to 
the alterations in the conditions attendant upon the nutritive 
processes. 
_ The economical acquisition of nutritive substance in proper 
____Given in an address before the Botanical Club of The University of Chicago, 
January 18, 1897. : ; 
