384 



PLANT MORPHOLOGY 



Metachlamydeae. In anatropous ovules having two integuments, the 

 outer one is united on one side with the funiculus. 



The development of the ovule is eusporangiate. Generally a single 

 hypodermal initial is differentiated at the apex of the nucellus, but occa- 

 sionally there are two or more initials, especially among the lower families 

 of Archichlamydeae. Ordinarily the initial, by a periclinal division, gives 

 rise to an outer primary parietal cell and an inner primary sporogenous cell, 



as in the microsporangium (Fig. 329). The 

 parietal cell may divide periclinally once or 

 twice again, or it may remain undivided. 

 In practically all the Metachlamydeae, and 

 exceptionally in the two other groups of 

 angiosperms, wall tissue is eliminated, the 

 hypodermal initial functioning directly as 

 the megaspore mother cell (Figs. 330A and 

 333A). In all other angiosperms the pri- 

 mary sporogenous cell is the megaspore 

 mother cell. It gives rise, by two successive 

 divisions, usually to a linear tetrad, the re- 

 duction in the number of chromosomes tak- 

 ing place at this time (Fig. 330B-D). A 

 T-shaped arrangement of the megaspores is 

 not infrequent. 



The Female Gametophyte. As in gym- 

 nosperms, the female gametophyte (embryo 

 sac) develops within the tissues of the ovule 

 and similarly is nearly always formed by the 

 innermost megaspore, the other three degen- 

 erating. The functional megaspore greatly 

 enlarges, encroaching vipon and absorbing the 

 abortive megaspores as well as more or less of 

 the surrounding nucellar tissue (Fig. 330E). Typically the megaspore 

 nucleus gives rise to eight nuclei by three successive divisions (Figs. 

 S30F-H and 331). Thus, as in gymnosperms, the development of the 

 female gametophyte is initiated by free-nuclear division, but in angio- 

 sperms the nuclei are almost always definitely eight in number. There is 

 no wall formation at this stage, but the free nuclei exhibit a striking 

 polarity, four being at one end of the embryo sac and four at the other 

 end. This polarity is established after the first nuclear division, when a 

 large vacuole appears between the daughter nuclei (Fig. 330G). 



One nucleus from each polar group now comes to the center of the 

 embryo sac. These two nuclei, called polar nuclei, come in contact with 

 each other and generally unite at once to form the fusion nucleus, or some- 



B 



Fig. 329. Early development 

 of the megasporangium of the 

 willow (Salix), showing single 

 hypodermal initial (A) and the 

 two cells derived from it (B): 

 the primary parietal cell (outer 

 shaded one) and the primary 

 sporogenous cell. (After 

 Chamberlain.) 



