REPRODUCTION OF THE ANGIOSPERMIA.—CROSSING. 843 
fore, is the true rudimentary embryo. Other variations occur 
in the mode in which the germinal vesicle (oosphere) is developed 
into an embryo, but the above is a general sketch of the sub- 
ject, and all that our space will aliow us to give. 
The changes which take place in the ovule during the deve- 
lopment of the embryo, and the subsequent growth of the latter, 
have been already generally alluded to when treating of the 
seed. 
Darwin, Sprengel, Hermann Miiller, Fritz Miiller, and 
others, have shown that, in numerous plants, crossing is neces- 
Fie. 1170. 
Fie. 1171: 
Fig. 1170, Vertica’ section of an ovule of a species of Enothera. ¢t. Enlarged 
end of the pollen-tube containing fovilla, which has entered the micro- 
py’e, and is seen } ressing inwards the apex of the embryo-sac. s, 7, Im- 
pregnated germinal vesicle, which already begins to exhibit two parts : 
one, the upper, forming a suspensor, s; and another, below, 7, a globular 
body, which ultimately becomes the embryo. e. Endosperm cells. 
Fig. 1171. Section of an ovule of a species of O7chis. t. Enlarged end ot 
the pollen-tube containing fovilla, which has passed through the micro- 
pyle, and is closely applied to the embryo-sac, the upper side of which it 
has pushed inwards. e. Germinal vesicle in the interior of the embryo- 
sac in an impregnated state, and dividing into two portions, the lower of 
which is the rudimentary embryo, and the upper forms a suspensor. 
sary for a completely fertile union of the sexes; that is, that the 
ovules of one flower must be fertilised by pollen from another 
of the same species. This may be effected in many ways; e.g. 
by the wind, when plants are spoken of as being anemophilous ; 
or frequently by the unconscious agency of insects, when they 
are said to be entomophilous, as in the Orchidacez, where the 
various modifications of structure to ensure cross-fertilisation by 
this latter means are strikingly beautiful. The observations of 
Darwin, Miller, and others have shown that self-fertilisation is 
probably exceptional in plants; certainly occasional crossing 
seems to be necessary. The term protandrous is applied to 
those plants in which the pollen is ripe and extruded before the 
