SEXUAL REPRODUCTION. 



297 



ceases to grow. With suitable protection and food-supply 

 it is then cast off as a seed (see further ^[ 408), and usu- 

 ally after a dormant period continues its development until 

 mature. 



403. In gymnosperms. — The growth of the embryo from 

 the egg in the gymnosperms stimulates the whole gameto- 

 phyte. This grows as rapidly as the embryo, which pushes 

 its way into it and remains completely surrounded by it (fig. 

 339). The whole ovule is also stimulated to growth. The 

 sporangium increases for a time, but is so crowded between 

 the growing gametophyte within and the hardening integu- 

 ment without that it is mostly absorbed (fig. 339). The in- 

 tegument grows for a time 

 to accommodate the struc- 

 tures within, but its tissues 

 finally become in whole or 

 in part thick-walled, form- 

 ing the seed- coat. In a few 

 gymnosperms (Cycas) its 



h 



Fig. 339. Fig. 340. 



Fig. 339. — Longitudinal section of the seed of silver fir (Abies pectinata), showing 

 straight embryo with several primary leaves in center of the endosperm (dotted) ; 

 m, the micropyle. The integument has become the testa (shaded with radial lines). 

 Between the testa and endosperm are the remains of the sporangium. Magnified 

 about 5 diam. — After Kerner. 



Fig. 340.— Longitudinal section of seed of Cycas circinalis. h, hilum (scar of attach- 

 ment) ; m, micropyle ; z, outer fleshy layer of integument ; ii and z'zY, two hard 

 layers of same ; s, thin cap-like remnant of sporangium ; />, gametophyte enlarged 

 forming the endosperm ; 0, eggs which failed of fertilization ; em, embryo produced 

 by a fertilized egg. Two thirds natural size.— After Luerssen. 



outer parts become fleshy, and the seed looks like a large 

 cherry. In the yew a second fleshy integument (an aril) 

 grows up around the hard seed (fig. 247). At maturity the 

 seed of gymnosperms thus consists of the embryo within 

 (fig. 340, em) surrounded by the gametophyte, p, whose cells 



