442 



THE VAOCULAR PLANTS 



from the strobili of lower 

 plants only in that the 

 megasporangia grow within 

 a closed organ, — the ovary. 

 Thus in flowers we find 

 megasporangia hidden. 

 This is new. Heretofore we 

 have found them borne 

 upon the surface of the 

 megasporophylls. It was 

 not necessary to cut open 

 any inclosing structure in 

 order to find them. But 

 now, among all angio- 

 sperms, we find that the 

 carpels (megasporophylls) 

 do not bear the ovules 



(megasporangia) upon their . surface ; they inclose them 



completely in a structure called the ovary. (See Figure 



228.) A new task has been given to the pollen tube. Its 



journey has been extended. It must now 



penetrate more than the tissue of the 



ovule in order to reach the egg ; it must 



penetrate also the tissues of the carpels 



which inclose the ovules. The carpels 



are usually united at the base into a 



compound ovary and extend upwards in 



the form of style and stigma, and all 



these tissues must be penetrated. The 



pollen tubes grow down from the stigma, 



through the style, and penetrate into the 



ovules at last. (See page 269.) 



Fig. 228. — Cross section of a young 

 ovary of lily. There are two vertical 

 rows of ovules in each of the three 

 carpels which compose this ovary. 

 The diagram indicates a single mega- 

 spore within each of the ovules shown. 

 The nucleus of this megaspore divides 

 and redivides until an eight-celled 

 female gametophyte results, such as is 

 shown in Figure 230. 



Fig. 229. — An ovary 

 of lily shortly after 

 fertilization has oc- 

 curred. Figure 228 

 represents a cross 

 section of such an 

 ovary when younger. 



