4 8 



SEX AND HEREDITY 



in Land-Plants the egg is never shed. It is retained by 

 the parent. In the Mosses and Ferns it is embedded 

 in the flask-like archegonium, where it is still accessible 

 to the free- swimming spermatozoid through the open 

 channel of the neck. Thus mechanical protection of 

 the egg is ensured, as well as the nourishment of the new 

 germ by the parent after syngamy. In the Flowering 

 Plant this protection and nutrition of the germ is still 



A . />> 



FIG. 30. 



A , Transverse section of the pistil of Caltha, showing the ovules protected in the 

 separate, pod-like carpels ; B, transverse section of the ovary of Lily, showing 

 the carpels united (syncarpous), closely enveloping the ovules. In both cases the 

 structure of the ovules themselves is substantially as in Fig. 25. 



more effectively secured. It is noteworthy how deeply 

 seated are the ova, and protected by successive coats of 

 tissue. First they are covered in by the carpellary wall 

 (Fig. 30) ; then by the integuments of the ovule ; next follows 

 the tissue of the nucellus ; and lastly the primordial ovum 

 is contained in the embryo-sac, which lies centrally in 

 the ovule (Fig. 25). Such repeated sheaths give a very 

 perfect protection against exposure to the air, or to 

 mechanical damage. But they increase the difficulties 

 of fertilization. These are overcome by means of the 

 exactly directed growth of the pollen-tube. On their 



