292 MORPHOLOGY OF THE ANGIOSPERMS 



Megaspore Mother Cells 



The megaspore mother cells are much larger than the surrounding 

 cells, with very large nuclei, dense cytoplasm, and thin walls (Fig. 

 102C). It has been generally implied in descriptions that the presence 

 of more than one or two megaspore mother cells in an ovule is rare and 

 restricted to a few primitive taxa — the Casuarinaceae, some of the 

 Amentiferae and Ranales — but many of the lower families have two to 

 several, and even some of the higher dicotyledons may have two. Primi- 

 tive genera of the Rosaceae and Ranunculaceae, some of the Asclepia- 

 daceae, even the Compositae, have more than one megaspore mother 

 cell. Most monocotyledonous genera have only one; a few have more, 

 and probably none has a cluster. 



Megasporogenesis 



Meiotic divisions in the spore mother cell form megaspores in 

 tetrads. The plane of wall formation is usually transverse, and the 

 spores lie in a linear cluster (Fig. 112A, B, C). Division in other 

 planes occasionally forms clusters of different shapes; T-shaped and 

 inverted T-shaped groups are occasional. There is much greater uni- 

 formity of arrangement in the tetrads of megaspores than in those of 

 microspores. The first division of the mother cell, probably always 

 transverse, forms daughter cells called dyads. Similar division in the 

 dyads forms the tetrad of spores. In the formation of the dyads and 

 of the tetrad of spores, walls are commonly formed separating the 

 individual cells; where walls are not formed, the protoplasts are united 

 within a common wall. In the typical tetrad, one of the four spores 

 enlarges and becomes the embryo-sac mother cell (Fig. 112C). Its wall 

 becomes the embryo-sac wall. The other three spores degenerate more 

 or less rapidly. The surviving spore is usually the chalazal member of 

 the tetrad; occasionally — Biitomus, Canna, StypheJia, Loranthns, some 

 of the orchids — it is the micropvlar member; only rarely is it one of 

 the median members — Piitoria, Arisfolochia. 



THE EMBRYO SAC 



Development of the embryo sac begins by germination of the mega- 

 spore, accompanied by enlargement of the spore and increase in cyto- 

 plasmic content and in size of the nucleus. Increase in size and content 

 is, in part, at the expense of the other spores, which are crushed and 



* Description of the embryo sac in detail can be found in the full and excellent 

 treatment by P. Mahcshwari, to which the reader is referred: "An Introduction to 

 the Embryology of Angiosperms," New York, 1950. 



