The Life of the Plant. 195 
Fertilisation is easier between different varieties of the 
same species than between different species, the result being 
in this case termed a varzety-hybrid. The stamens are often 
replaced in them by petals, thus forming what are called 
double sterile flowers, on which account they are not gene- 
rally so well calculated for propagation as species-hybrids. 
It is easily understood that agriculture, and especially 
horticulture, takes advantage of these peculiarities in order 
to call into existence varieties of plants cultivated for their 
usefulness or their beauty. 
The subject of fertilisation is incomplete without a description of 
the mode of reproduction of Cryptogams by means of oospheres. But as 
an account of the very numerous modifications of this process must 
necessarily be given under the separate classes, the whole subject is de- 
ferred till the section on classification. 
PHENOMENA OF MOVEMENT. 
Motion is an essential condition of life. The plant, 
therefore, like the animal, exhibits phenomena of motion ; 
but since the former finds conditions suitable for existence 
on all sides, these phenomena are in general less strongly 
manifested than in the case of the latter. It is not neces- 
sary here to allude to those movements of a passive 
cnaracter which the plant performs under the influence of 
wind or wave, or when bent to the ground beneath the 
weight of its fruit, msing again subsequently to an erect 
position ; in short, to those movements which are imparted 
to it by the obvious action of mechanical forces. Those 
only will be referred to which are inseparable from life, 
and are a direct result of the vital processes. 
It is impossible to give a minute account of all these various pheno- 
mena of movement; but they will at least be made somewhat more 
rotundifolia, M. sylvestri-rotundifolia the result of the fertilisation of JZ, 
rotundifolia by M. sylvestris.—ED. | 
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