. Evolution of plants . PLANTS compound spore-sac, or synangium. In An-giopteris and one other closely allied genus, this isnot the case; the sporangia are very closelyassociated in two ranks to form each sorus,but are not actually joined. Even here, how-ever, the sporangia are different from those ofmost Ferns; they are largerand of more solid construc-tion, with a wall severalcells thick. The opening ofthe sporangium to set freethe spores is effected bymeans of a patch ofthickened cells at the apex,an arrangement bearingsome resemblance to thatin the Osmundas. In theother three genera, Ma-ra
. Evolution of plants . PLANTS compound spore-sac, or synangium. In An-giopteris and one other closely allied genus, this isnot the case; the sporangia are very closelyassociated in two ranks to form each sorus,but are not actually joined. Even here, how-ever, the sporangia are different from those ofmost Ferns; they are largerand of more solid construc-tion, with a wall severalcells thick. The opening ofthe sporangium to set freethe spores is effected bymeans of a patch ofthickened cells at the apex,an arrangement bearingsome resemblance to thatin the Osmundas. In theother three genera, Ma-rattia, Dancea and Kaul-fussia, including the great under surface of Kaul- the sporangia are frond, sWng cir- jn MaraUia tney form pod-cular synangia. After Jan- ° * shaped fruits or synangia, each containing a double row of compartments (of which there are about ten in each row), representing the sporangia (see fig. 17, A). In one species the whole compound body is stalked, and closely resembles a compound. gler and magnified. THE FERNS 151 pollen-sac of Bennettites. When ripe, the twokidney-shaped halves of the fruit split apart, likeopening a book, and then each of the numerouscompartments in either half releases the spores bymeans of a slit on its inner surface (fig. 17, A).Danoea has much the same arrangement, exceptthat the spore-fruit does not split open and thespores escape through pores instead of slits. Thisgenus shows some slight difference between sterileand fertile fronds, which are not usually distinctin the family. In Kaulfussia, which has fronds like the leavesof a Horse-chestnut, the synangia scattered onthe under side are circular, each having the formof a neat rosette, with a cup-shaped hollow in themiddle, into which the compartments (about adozen in number) open by slits (see fig. 17, B). These curious spore-fruits are no doubt an ad-vanced form of fructification compared with thesimple spore-sacs of the ordinary Ferns. Yet thef
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