The L ufe of the Plant. 18 s 
visit only particular plants, the pollen is not, as a rule, 
wasted by being carried to a flower of a different species. 
Too great importance cannot be attached to this function of 
insects ; and many exotic plants, such as the vanilla, were 
long cultivated in our hot-houses without producing fruit, 
because the insects required for their fertilisation were want- 
ing, until their place was supplied by artificial pollination. 
In particular cases in which fertilisation would otherwise be impos- 
sible, other special contrivances are found, especially in water-plants. 
When the pollen-grains come into contact with water, with a very few 
exceptions (as in Ceratophyllum and Zostera), they absorb so much as 
to cause them to burst, or to occasion the escape of their contents from 
one of theig pores so quickly that they perish. In order to prevent 
this injurious effect it is almost invariably the case that submerged plants 
raise their flowers above the water, as in Sagzttaria and Hydrocharts. 
In Utricularia (Fig. 356), the bladders on the leaves which are pre- 
viously empty, become filled with air at the time 
of fertilisation, raising the whole plant to the sur- 
face of the water, while it again sinks at the close 
of the period of flowering. In Zéatine and Alismia 
air-bladders are formed, by a vital activity not yet 
accurately investigated, between the connivent 
stamens, within which fertilisation can be accom: Fic. 356.--s_ Bladde 
plished undisturbed. The most remarkable case fottaiec Be (em 
is, however, presented by the dicecious Vallisnerta 
spiralis, which occurs abundantly in marshes in South Germany. The 
male flowers are seated on very short pedicels at the base of the leaves, 
often several feet below the surface of the water; the female flowers 
on the contrary on very long pedicels, which at a particular time 
become greatly elongated and raise the flowers to the surface of the 
water. The male flowers then become detached from their pedicels, 
rise to the surface, are floated among the female flowers, and fertilise 
them. After this has been accomplished the pedicel of the female flower 
coils up spirally, and the fruit ripens beneath the water. 
Notwithstanding that appearances would seem to point to the Hist 
being always fertilised by the stamens which surround it, more exact 
observation has nevertheless shown that seéf-fertlisation of this nature 
does not, as a rule, take place, but that on the other hand cross-fertilisa- 
tion, 2.€. a crossing between different flowers on the same plant, or 
between flowers on different plants of the same species, is much more 
