FORMATION OF SEEDS. 
21 
and that the bark retires back for the purpose, as I showed in a 
former letter. Whether the bark is pushed back by the violence 
of the rising sap, (which I am inclined to think,) or that it 
retires back of itself, the effect is still the same. The sap rises 
between the two cylinders of the wood and bark, but I must 
observe, that this process is wholly separate from the usual flow 
of the sap in the wood, and belongs to the alburnum vessels 
only. The liquid is not wholly loose ; vessels, composed of a 
narrow-twisted band, produce the exterior of the cylinder, and 
this is the cause of the various changes observed in the appear- 
ance of the alburnum : sometimes it looks like tiles laid on 
each other; sometimes like a twisted vessel, laid almost flat ; 
but each varying form is owing to the pressure of the sap on the 
divided foldings of this curled ribbon. It must be remembered, 
that it is a vessel within a vacancy ; therefore, the sap is doubly 
secure : but this very circumstance causes the jelly-like matter 
to assume folds and creases, difficult to understand, till the cause 
isknown. When the sap has, in its liquid state, deposited all the 
seeds, and lodged them in the buds, then by degrees having 
cleared itself of them, it coagulates, and becomes a jelly. In 
this state it remains all the winter , and when the spring arrives, 
and the bark begins to retire, the bud of the preceding year 
completes itself, by forming its calyx and flower stalk, which it 
never does till the seeds have entered the corolla. But before 
the flower developes, another process takes place, which I 
might also announce as a late discovery, having only hinted at 
it before. It is the producing the pollen. When the corolla xhe forming 
is in its cradle, the cases of the stamen are there, but no pollen, the pollen. 
The powder of the males is formed by some mixture, which 
requires the dews to perfect it, some liquid from above, which 
is deposited on the top of the plant, conveyed into the interior 
by the hairs, and thus communicated to the higher juices. Take 
a branch of any tree, before it flowers, when the buds are still 
at the bottom of the peduncle, and cut open the whole, dividing 
it down the middle, and a yellow matter will be found at the 
top of the wood, where the pith stops, whose tainted appearance 
indicates something extraordinary ; place a very thin slice of 
this in the solar microscope, and the yellow matter will be dis- 
covered to be the pollen , protruding in a distinct manner : nor 
is this all ; the flowers, after being aggregated, begin to divide, 
then 
