£02 Mr. Correa de Serra on tht 
substance which surrounds these seeds can be considered as 
true pollen. 
Pollen is by its nature immiscible with water, and specifi- 
cally lighter than that fluid. In the aquatic plants, which have 
a farinaceous pollen, the buds of the flowers emerge from the 
surface of the water, and the fecundation is performed in the 
open air. The phasnomena attending the blossom of the Pota- 
mogetones, Myriophylla, Vallisneria, &c. are too well known 
to require a particular mention. In some aquatic plants, whose 
flowers have not the faculty of emerging from water in the pe- 
riod of fructification, but still are endowed with farinaceous pol- 
len, nature has taken every precaution to defend it from that 
element. The flower of the Zostera is situated, and its fecunda- 
tion happens, in the interior cavity of the stem, which opens 
itself afterwards to let loose the fecundated seeds. The concave 
bases of the leaves in the Isoetes, closely adhering to each other, 
and perhaps more so in the act of fecundation, forbid the en- 
trance of water to the minute flowers situated within them. 
In the Pilularia, and Marsilea, whose flowers are exposed to 
inundations, the fecundation is performed in perfectly closed 
vessels. Even in plants living in the air, nature employs 
numberless well known contrivances, to shelter the farinaceous 
pollen from the contact of water in rainy seasons. 
The pollen, to be active in fecundation, needs not al- 
ways be farinaceous. In most Apocyneae it is rather a 
fluid ; in the Orchideae it is an aggregate of solid parts, of a 
ceraceous appearance ; in some Contortae it is found in a solid 
or rather viscous state. In the Pilularia, and Marsilea, the par- 
ticles of the pollen are kept in small bags of a mucous 
