126 A TEXT-BOOK OF BOTANY. 



which after fertihzation the endosperm is derived, The embryo- 

 sac, as it is organized at this stage, constitutes what is regarded 

 as the female gametophyte (Fig. 82). The undifferentiated 

 embryo-sac constitutes the megaspore,. which latter, after germina- 

 tion or differentiation into egg-cell and other cells, constitutes the 

 gametophyte. It is thus seen that in the female gametophyte 

 of the Angiosperm archegonia are apparently not formed. The 

 gametophyte, then, consists of the cell group containing the tgg 

 and the remaining portion of the embryo-sac, which latter may 

 be compared to a prothallus. This comparison is not difficult to 

 understand if we bear in mind the structure of the gametophyte 

 in the Gymnosperms, and particularly if we recall the structure 

 in Selaginella and other higher Pteridophytes. 



Fertilization. — \\ hile in the Gymnosperms the pollen grains 

 are usually provided with wings so as to bring about their trans- 

 ferral to the carpel by the agency of the wind, in the Angiosperms, 

 on the other hand, the grains are not provided with wings, but 

 are adapted to the transferral by insects. Pollination, however, 

 may be also effected by the wind, as is the case with many of our 

 forest trees. After the deposition of the pollen grain on the stigma, 

 the tube-cell begins to form a tubular process (pollen tube) which 

 carries the male nuclei to the egg-cell (Fig. 83, i). It pierces 

 the tissue of the stigma (Fig. 83, h) and traverses the style (Fig. 

 83, g) until it reaches the micropyle of the ovule, which it enters 

 (Fig. 83, 111), then reaching the nucellus it penetrates this, enter- 

 ing the embryo-sac. The tip of the tube breaks and one of the 

 generative nuclei which has been carried downward unites with the 

 egg, after which a wall is formed, giving rise to an oospore. The 

 oospore develops at once into the embryo or plantlet as seen in 

 the seed, this stage being follo\yed by a period of rest. In fact, 

 the young plant may lie dormant in the seed for years. 



Development of Seed. — The steps in the development of 

 the mature seed occur in the following order (Fig. 84). The 

 oospore divides into two parts, an upper portion which gives rise to 

 the embryo, and a lower portion w^hich by transverse segmentation 

 gives rise to a short suspensor (Fig. 84, v) which practically serves 

 the same purpose as in the Gymnosperms (page iii). The em- 

 bryonal cell develops the embryo, which consists of : (l)a root por- 



