122 STRUCTURAL BOTANY 



towards the placenta. The embryo-sac takes part in 

 this curvature (see Fig. 44, 9, and Fig. 43). Ovules 

 in which the nucellus and embryo-sac are bent in this 

 way are called campylotropous. 



The structure of the ovule, then, when ready for 

 f ertilisation, is as follows : it is attached to the placenta 

 by a stalk, the funicle ; the curved body of the ovule 

 is enclosed by the two integuments, which leave a 

 narrow opening at the free end, the micropyle ; the 

 interior of the ovule is almost entirely occupied by the 

 long curved embryo-sac, which is in immediate contact 

 with the inner integument. The rest of the tissue of 

 the nucellus has disappeared except at the base. 

 Within the embryo-sac, at the micropylar end, are 

 three cells, the egg-apparatus. The most deeply- 

 seated of the three is the ovum. In the protoplasm 

 of the sac is a large nucleus, which has been formed 

 by the fusion of the two polar nuclei. We shall prob- 

 ably find no trace of the antipodal cells, which at an 

 earlier stage occupied the chalazal end of the sac. 



The description just given, though taken specially 

 from the Shepherd's Purse, applies to the ovules of the 

 Wallflower also, except that the latter usually has 

 several embryo-sacs side by side. Only one of them, 

 however, contains an ovum capable of being fertilised. 

 Fig. 43 is drawn from an ovule in which only one 

 embryo-sac is visibla 



e. Fertilisation 



We have next to learn how fertilisation is brought 

 about The ovum by itself is quite incapable of any 



