DIV. n 



SPERMATOPHYTA 



545 



base of the axis (Fig. 510). The microsporangia are borne on the 

 lower surface of the sporophylls, two or more being present on each. 

 Their opening is determined as in the sporangia of the Pteridophyta 

 by the peculiar construction of the outer layer of cells of the wall 

 (exothecium). The pollen grains are spherical, and are frequently 

 provided with two sacs filled with air, which increase their buoyancy 

 and assist in their distribution by the wind (Fig. 510). On germina- 

 tion the outer firm layer of the wall of the pollen grain (exine) is 

 completely lost, being fractured by the increase in size of the proto- 

 plasmic body. 



In many Gymnosperms the FEMALE FLOWERS or CONES resemble 

 the male flowers in being composed of an axis bearing numerous 

 * spirally-arranged sporophylls. 



In other cases they differ 

 A from this type in various 



ways, which will be described 

 below (pp. 589 ff.). 



FIG. 511. Flower of Paeonia peregrina, iu longitudinal 

 section, k, Calyx, and c, corolla, together forming 

 the perianth ; , androecium ; g, gynaeceum. The 

 anterior portion of the perianth has been removed, 

 (i nat. size. After SCHENCK.) 



Fio. 512. Flower of Acorus Calamus, 

 pg, Perigone ; a, androecium ; g, 

 gynaeceurn. (Enlarged. After 

 ENGLKR.) 



In Angiosperms, on the other hand, a union of micro- and macro- 

 sporophylls in the one flower, which is thus HERMAPHRODITE, and the 

 investment of the flower by coloured leaves (differing in appearance 

 from the foliage leaves), forming a PERIANTH, is the rule (Figs. 511, 

 512). The Querciflorae afford an example of an exception to these 

 statements. In contrast to the UNISEXUAL or DICLINOUS flower with 

 the sporophylls arranged spirally on an elongated axis, which is 

 characteristic of the Gymnosperms, the perianth leaves and sporo- 

 phylls in the Angiosperms are usually borne in whorls on a greatly 

 shortened axis. THE ARRANGEMENT OF THE FLORAL LEAVES IN 



WHORLS, THE COLOURED PERIANTH, AND THE HERMAPHRODITE NATURE 

 OF THE FLOWERS ARE THUS CHARACTERISTIC OF ANGIOSPERMS, although 



these features do not apply without exceptions to all angiospermic 

 flowers. These differences depend on the important factor of the 

 MEANS OF POLLINATION. When, as is the case with the Gymnosperms 



