314 PRINCIPLES OF BOTANY, ETC, 
the exhalant vessels, which seem to contribute a 
great deal to the condensation and proper prepara- 
tion of the sap. They indeed grow themselves at 
the very period, they perform these functions. When 
therefore the seed has attained its proper size, the 
vessels of the pappus become plugged up, and it re- 
mains dry upon the seed. 
§ 289. 
The stigma, now in its state of puberty, or when 
fit for impregnation, becomes covered with a fluid, 
which Koelreuter likewise considers as oily, but of 
the nature of which we know in fact very little. 
The period when the stigma is moist and the an- 
thers burst, is the period of impregnation. This 
operation, however, is in plants performed in so 
very striking a manner, that we must be astonished, 
when we find how truly wonderfully nature has pro- 
vided for all this. Most flowers are hermaphro- 
dite, or such as have both male and female organs 
of generation, and one would from this circum- 
stance be led to believe, that in such flowers ime 
pregnation would be immediately completed; but 
it happens otherwise. 
Mr Sprengel has made numerous observations 
and experiments on this point, most of which are 
highly important. He discovered two principal 
ways in which seeds are impregnated, to wit, Dicho- 
samy, (Dichogamia), and Homogamy, (£lomogamia). 
He calls it Dichogamy, when in a hermaphrodite 
flower one organ of generation is first evolved, and 
after it has lost its fecundating power, another ge- 
nerative 
