23 
reacting bases are organically connected only by means of tissue which 
must completely have ceased to grow (it is not available for measure- 
ment), and the harmony of the entire reaction is no evidence of any 
communication between the units concerned in it.% 
Anatomy of the leaf—In describing the anatomy of the coconut leaf 
nothing need be said about the fibro-vascular tissue except that the finest 
longitudinal veinlets are hardly more than 0.1 millimeter apart, so that 
water in order to reach any cell of the parenchyma only needs to pass an 
exceedingly short distance by osmosis. The structure of the individual 
veins and veinlets offers no peculiarities. 
The most striking structure in the leaf is what may be called the 
“hinge.” Running ventrally for its entire length along each side of the 
midrib of the pinna is a narrow strip, sharply differentiated from any 
neighboring living tissue by its colorless contents. A crease along the 
middle of each of these strips makes the leaf thinner at this point than 
anywhere else, the colorless hinge tissue occupying more than half the 
thickness of the leaf but not entirely crowding out the green mesophyll. 
The epidermis of the hinge, as seen in transverse section, is remarkable . 
for its exceedingly convex outer walls. The two accompanying figures 
(15 and 16) make this structure clear. 
Because of the convexity of the outer walls of the individual cells, the 
wall of the epidermis, in this situation, as a whole is very much wrinkled ; 
so that a bending or even a stretching can obviously be accomplished by 
a very slight and easy bending of walls at right angles, without giving rise 
to the uncompensated stretching of any one unit. Other parts of the 
leaf have the thick outer walls practically plane, and as any bending would 
involve the extension or direct compression of the whole of one of them 
these parts are practically rigid. Therefore, the crease mentioned above 
facilitates movement not only because it makes the leaf thinner at this 
point but also because it increases the convolution of the walls and reduces 
their resistance. 
The active tissue concerned in the movements of the hinge is the color- 
less mesophyll. Its cells are large, and they have thin walls which are 
easily bent or even stretched. It is without intercellular space, so that 
the slightest alteration in the volume of the individual cells changes that 
of the entire tissue. The volume of the cells must obviously vary with 
their water content. When the leaf is well supplied with water the cells 
of the hinge are distended to their full capacity and it is open, thus hold- 
ing the two sides of the pinna as far apart as possible. When the supply 
of water is insufficient the reverse takes place. By this means the expo- 
sure of the pinna to the rays from the sun or sky is lessened and a “dead 
air” space, though usually a very imperfect one, is formed under it. In 
both of these ways the further loss of water is checked. 
When the pinna is losing water faster than it is being furnished from 
“Cf. my paper, “The Geotropism of Split Stems,” Bot. Gaz. (1900), 29, 189. 
