206 DEVELOPMENT OF THE OVULUM 
merely minutely papillose on the upper surface. The nervules 
pass from the apex of the petiole in a radiating manner. On 
either side of each is a narrow canal containing air, which 
- communicates directly with those situated under the callous 
disc, and through these with the stomata. There is likewise 
free communication between the above, and the air cavities 
- of the petiole. 
. The space under the disc is divided into a number of irregu- 
lar cavities communicating with one another, and on their pa- 
rietes globuline is found. The cavities on either side of the 
nervules appear to have no communication whatever with 
the parenchyma of the leaf. This structure is hence remark- 
able for the curious site of the stomata, and for isolation of the 
green parenchyma from the organs of aération. The stomata 
are evidently open. See pi. 56, E. figs. 3, 4, 5, 6, 7. 
The pollen consists of two very distinct membranes, the 
outer is of a pale, lemon, yellowish-brown colour, thick, not 
extensible, and marked with three longitudinal lines: with 
variously shaped depressions. See pl. 56, E. fig. 9 
Are these the boundaries of cells, or are they rather glands 
filled with oil ? globules, which are of a lemon colour, separate 
on pressure, 
The inner is colourless, highly extensible, containing 
very minute granules. Immersed in water, the outer coat 
bursts suddenly along one of the longitudinal lines, and 
expels the inner membrane without altering its shape. To 
a portion of the circumference of this, lax cellular substance 
adheres, which perhaps indicates its attachment to the external 
membrane. Dehiscence from stigmatic influence is much_ 
more gradual; the inner membrane being elongated, and at 
first presenting an hour-glass shape. The upper end of this 
is covered by the outer membrane. ^ See pl. 56, E. figs. 
2and8.  . 
The ovula are always solitary and pendulous. During 
their development no change in situation occurs, and con- - 
trary to a very general law, the openings of the coats are 
always turned away from the stigmatic canal. This is open 
