C VGA DEM. 



505 



(as in the sporangia of Lycopodiaceag, Equisetaceae, and Ophioglossaceae) into an 

 outer layer of smaller cells enclosing a larger-celled tissue ; the cells of the latter 

 continue to grow and divide in all directions, and the mother-cells of the pollen 

 are finally isolated, but densely crowded together, as in Dicotyledons. The mode 

 of division of the mother-cells is nevertheless more like that of Monocotyledons in 

 this respect, that they first of all divide into two daughter-cells, each of which 

 again undergoes bipartition. The first division- wall is partially formed, .^s in Dico- 

 tyledons, by the slow growth of an annular ridge of cellulose, formed in the depres- 



FlG. ^^^.—Zainia nmricata (after Karsten). A a male flower (natural size) ; B transverse section of one; C one 

 of its stamens with the pollen-sacs x and the peltate expansion s (seen from below) ; D the upper part of a female 

 flower (natural size); E transverse section of one, s the peltate scale bearing the ovules s k; ^longitudinal section of 

 a ripe seed ; e endosperm, c cotyledons, x the folded suspensor. 



sion produced by the previous constriction of the protoplasm of the mother-cell ; 

 but in each of the two daughter-cells the second partition appears to be formed 

 simultaneously, as in Monocotyledons. The four young pollen-cells are now freed 

 by the rapid absorption of the cell-wall which surrounds and separates them. The 

 pollen-grains, when free from their mother-cells, are unicellular and spherical; but, 

 during their further growth, the contents, enclosed by an extine and intine, divide 

 into two cells, a smaller and a larger one, each possessing a nucleus. The smaller 

 of these two cells, lying on one side against the intine of the pollen-grain, becomes 

 arched on the opposite side, and projects in the form of a papilla into the larger 



