PRINCIPAL GROUPS OF PLANTS. 



125 



m 



FIG. 82. Development of embryo-sac or megaspore in an Angiosperm. la, longi- 

 tudinal section through a young ovule. Ib, longitudinal section through a rudimentary 

 ovule before the formation of the integument, showing mother cell of the embryo-sac (mega- 

 spore) (em) and primary tapetal cell (t). II, later stage showing the two cells into which 

 the mother cell has divided, the nuclei of which are in the act of dividing. Ill, mother- 

 cell of the embryo-sac divided into four cells (sporogenous mass of cells) ; the lowest of these 

 cells (e) displaces the rest and becomes the embryo-sac in IV. IV, pek, is the primary 

 nucleus of the embryo-sac. V, two daughter cells resulting from the division of the nucleus 

 of the embryo-sac. VI, VII, show egg apparatus composed of two synergids (s) and the 

 oosphere (o), and antipodal cells (g). VIII, longitudinal section through a mature ovule 

 with the inner integument (ii), the outer integument (ai), the nucellus (n), the vascular 

 bundle (gf) entering the funiculus (f), and secondary nucleus in the embryo-sac (sek). 

 After Strasburger. 



