320 PRINCIPLES OF BOTANY, ETC; 
cloud. The sulphur rain, as it has been called; 
which falls sometimes in spring, after thunder storms, 
is nothing else, but the pollen of the Pinus sy/vestris 
carried about in the air by wind. 
Such plants as have on one stem male flowers 
only, on another female flowers alone, are always 
provided with nectaries, and the male flowers are 
larger by far than the female, to allow more readily 
the insects to carry the pollen to their female 
neighbours. 
Vhe Valisneria spira4s, a water-plant of Italy, has 
the different sexes in different flowers; but here the 
male flower parts with the stem, and swims upon 
the water, that the aquatic animals may the sooner 
carry its pollen to the female plant. It is indeed a 
veneral rule, that all those aquatic plants which do 
not come under Linne’s 24th class, can in no other 
way be impregnated but above the surface of the 
water. 
Many foreign plants flower with us, having dis- 
linctly formed hermaphrodite flowers, but notwith- 
standing bear no seeds. The climate, however, is 
not always the cause of their batrenness, but the 
want of insects, which nature destined in their na- 
tive countries to fecundate their seeds, and which 
we have not, along with the plants, received into 
our gardens. One experiment will confirm the 
truth of this observation: Vhe Abroma augusta 
flowered for many years here, in Berlin, in a hot- 
house, where no insects had access, without ever 
bearing a single fruit. Vhe gardener tried the ex- 
periment to put the pollen, by means of a hair- 
brush, 
