ee =. ee ee a ee ee eS re ee ee i at a. ip tli 
25 
before had become chronic. ‘This was due to the prolonged drought and 
not to the age of the leaf, for in seasons of fair precipitation the pinnz 
lose none of their original power of expansion, at least until they are 
much older than the ones measured in these experiments. 
Aside from the hinge, the mesophyll is differentiated into the green 
assimilating, and what, in deference to custom, I will call a water-storing 
tissue. The latter is composed of two layers of cells, immediately beneath 
the upper epidermis. ‘The upper of these layers has the cells elongated 
at right angles to the length of the pinnz, as is shown by figs. 17 and 18, 
while those of the inner layers are considerably deeper and larger. Both 
are almost perfectly transparent and their contents consists almost 
entirely of water. ‘They form no inconsiderable part of the volume of 
the leaf. Their walls are not sufficiently thin either to be collapsible or 
to stretch very easily, so that only an insignificant part of the water which 
they contain can ever be available to replace any loss on the part of other 
cells, They are primarily rather to be regarded as a screen, the function 
of which is to mitigate the injurious effects of too extreme insolation on 
the underlying green cells. The same is true of the thick-walled, so- 
called water-storing tissue of many other plants. 
The green mesophyll is but feebly differentiated. There usually are 
two layers which may properly be termed palisade cells, and about four 
more, the cells of which are irregularly placed; but the leaf throughout 
is too compact for any tissue appropriately to be designated as spongy. 
The turgor of the assimilating tissue is equal to about 0.5 normal potas- 
sium nitrate. 
The cells of the epidermal tissue are approximately isodiametric in 
surface view, their least diameter being the depth. They are devoid of 
chlorophyll, and hyaline. Their turgor is about that of the hinge cells, 
but their heavier walls prevent any considerable variation in their water 
content. The outer wall of the upper epidermis is 6 to 7 » thick; that of 
the nether, 5y. ‘The surfaces are glabrescent. 
The stomata of the coconut leaf are confined entirely to the nether surface, 
where they number about 145 per square millimeter. They average in size about 
30 by 33 with an area for the pores and for both guard cells of 740 « square. 
As the accompanying drawing of the transverse section (fig. 19) shows, 
the stomatal apparatus is practically superficial, the back of the guard 
cell being sunk just enough to make room for a hinge in the wall outside 
it and to permit it to move without interference from the thick, outer 
wall of the epidermis. The mechanism is exactly that of Schwendener’s 
type of Amaryllis.° 1° The back wall of the guard cell is so thin that it 
collapses and wrinkles in plasmolysis. The ventral half of each, namely, 
the one next to the pore, is strengthened by the powerful ridges of 
* Schwendener: Ueber Bau und Mechanik der Spaltéffnungen, Monatsber. Akad. 
d. Wiss., Berlin, 1881, 833. 
** Copeland: The Mechanism of Stomata, Ann. of Bot. (1902), 16, 339. 
