FERTILIZATION IN GYMNOSPEKMS. 



437 



1130. The terminal cell of the suspens'or is followed by the 

 initial cell or cells of the embryo proper ; the different stages of 

 the development of the embryo can be traced in the ovule of one 

 of our most common weeds, Capsella (compare Figs. 203-205). 



The case above de-scribed is a simple one. but may serve as a 

 type of all normal cases of fertilization in angiosperms. the innu- 

 merable deviations from which cannot be further alluded to here. 1 



1131. With the changes in the embryo sac there are concomi- 

 tant changes in the whole nucellus and its integuments. A 

 certain amount of food of some kind (see 509) is stored either 

 in the sac or in the developing tissues around it, constituting 

 the so-called albumen of the seed. The food within the develop- 

 ing embryo sac is termed endosperm ; if around it, perisperm. 

 But the changes do not stop with the ovule as it ripens 

 into a seed; they go on also in the surrounding parts. In 

 fact, as soon as fertilization has begun, the flower wilts, and 

 in most cases the external organs fall. The ovarv, sometimes 



O / ' 



with associated parts such as the calyx, the receptacle, etc., 

 passes through changes by which it becomes the fruit. 



FERTILIZATION IN GYMNOSPERMS. 



1132. The chief differences between the reproduction in these 

 plants and that in those 



just described are in the 

 preliminary development 

 of the pollen and the 

 ovule. 



1133. Pollen of gy-m- 

 nosperms. The grain 

 is distinctly divided by 

 a curved partition into 

 two portions, and one of 

 these portions is fre- 

 quently divided in much 

 the same way into two 

 parts. Comparison of 

 this pollen with that of 



The student is urged to study with great care the masterly treatise by 

 Strasburger, Ueber Befruchtung und Zelltheilung, 1878, and the more succinct 

 account in his Practicum, 1884. 



FIG. 206. A, pollen-grains of Biota before their escape from the pollen-sac. /, fresh, 

 // and III swollen by water; the extine <- having split off, the protoplasmic contents 

 are seen. B, pollen-grains of Pinus pinaster before their escape from the pollen-sac; a 

 side and a dorsal view. (Sachs.) 



