PART I. ORGANOGRAPHY. 



embryo-sac. Sometimes the embryo-sac is so strongly devel- 

 oped as to entirely replace the nucellus, as in some Orchids and 

 in Monotropa hypopitys ; in other instances, its upper end 

 bursts through the tissues of the nucellus and either lies at the 

 upper surface of the latter, or protrudes beyond it. The latter 

 is the case with the commonly cultivated green-house plant, 

 Torenia Asiatica. Oftener, it is more or less deeply buried in 



Fig. 295. 



Fig. 296. 



Fig. 295. — Diagram of flower in vertical section to show fertilization of ovule: ca, 

 calyx; co, corolla; st, stamen; p, pollen grain sending tube, t, down through the tissues 

 of the style and into the micropyle of the ovule. In the nucellus of the ovule is seen the 

 embryo-sac with the synergidae, oosphere, antipodal cells and nucleus. 



Fig. 296. — Diagram representing an ovule in the process of fertilization: /", funiculus 

 of the ovule penetrated by a spiral vessel; ch, chalaza ; a, outer integument, or primine; 

 b, inner integument, or secundine; />, end of pollen-tube that has penetrated to the nucellus, 

 n; e, embryo-sac; nu, nucleus of embryo-sac; s,s, the two synergidae ; o, the oosphere; 

 and ac, the antipodal cells. 



the tissues of the nucellus. It is, however, always of large size, 

 and thick-walled as compared with the other ceils of the 

 nucellus. 



At the upper or micropylar end of the embryo-sac is formed 

 the egg-apparatus, consisting of two cells called the synergidce, 

 and another cell called the oosphere or germ-cell, which, after 

 fertilization, is destined to develop into the embryo. The syner- 

 gidae each have a vacuole or transparent sap-cavity at the lower 



