394 MacDougal . — The Mechanism of 
A clear comprehension of the results presented below necessi- 
tates the delineation of the general features of structure of the 
tendril of Passiflora coerulea . 
Structure of the Tendrils of Passiflora (IX). 
An examination of these organs was first made by Hugo 
von Mohl (XIII, p. 3, Figs. 4 and 5, PL II) ; he describes the 
structure, as seen in cross-section, as follows : ‘ Die Rinde 
besteht aus regelmassigen griinen Zellen (a) ; an der Stelle 
wo sie mit dem j ungen Holze (b) zusammenstosst, liegen die 
Bastrohren in getrennten Biindeln (c). Der Holzkorper besteht 
aus feinen Spiralgefassen und dickwandigen Holzzellen, die 
Markzellen (e) sind in die Lange gestreckt ; 3 and later by 
Worgitsky (XXII), in which work it is to be noted that chief 
attention was paid to the distribution of the tissues in 
cross-section. 
In longitudinal section the epidermal cells are somewhat 
uniformly rectangular in outline, with the long axis parallel 
to the long axis of the tendril. The protoplasts occupy 
a large proportion of the cell cavity. The nucleus, especially 
on the concave side, generally lies against the inner wall. 
The protoplasm is richly granular, but more markedly so in 
the concave side ; moreover the proportion of granular 
substance gradually increases in the cells of the concave side 
from the base to a point near the tip, in a manner corre- 
sponding quite exactly with the degree of irritability exhibited 
by the respective areas included. This fact may be demon- 
strated by staining longitudinal sections in eosin and haema- 
toxylin as I have previously shown (IX). When so prepared, 
the gradation in density is appreciable with a magnification 
of 50 diameters. The outer walls of the concave side are 
slightly arched outwardly, and are only cuticularized after 
maturity. These convexities must greatly increase the 
delicacy of perception of stimuli. Any solid body applied to 
their surface would in consequence touch the crest of these 
convexities, and the shock of impact, or pressure of contact, 
