366 PART IV. VEGETABLE TAXONOMY. 



Their alliance to the higher members of the preceding Series, 

 is especially shown in the fact, that an endosperm, equivalent to 

 the female prothallium of the heterosporous Ferns, is produced 

 in the embryo-sac previous to fertilization, and bears archegonia 

 in the same manner, and in the fact that the pollen-grains become 

 two or more celled before emitting a pollen-tube, and at least one 

 of the cells takes no part directly in the formation of the latter, 

 but behaves like, and is really the equivalent of, the rudimentary 

 male prothallium of Salvinia and Selaginalla. 



The Class includes three orders, which, in appearance and 

 habits of growth, differ widely from each other, but agree essen- 

 tially in their modes of reproduction. They are the Cycadea, 

 the Conifei-cz and the Gnetacece. 



(A) The Cycadeae. The plants of this order have much the 

 aspect of Tree-ferns with which, in fact, they are more strongly 

 allied than are any other members of their class, having un- 

 branching scaly stems crowned at their summit with ample 

 pinnate leaves. They are slow-growing plants which, though 

 once exceedingly abundant and playing an important part in the 

 world's flora, are now rare, consisting of only about seventy-five 

 species, and these confined to tropical and sub-tropical regions. 



The stems, though externally scaly and unbranching, like 

 those of Tree-ferns, more closely resemble in their internal 

 structure those of the Pines and Dicotyledons, having the woody 

 elements arranged in much the same way and growing in 

 substantially the same manner. Compared with the Pines the 

 medullary rays are broader, and the cells composing them thin- 

 ner-walled and the pith is usually large. The woody elements 

 consist of discigerous, scalariform or reticulate tracheids in the 

 secondary wood, with a few spiral tracheids in the primary wood 

 or medullary sheath, but true ducts are seldom found. The 

 stems also differ from those of Ferns, in being continued down- 

 ward into a tap-root. 



The leaves are arranged on the stem in compact spirals, and 

 consist of two kinds, the ample pinnate ones already mentioned, 

 and more numerous, scale-like, brown, leathery, rudimentary 

 ones. Every year or two the crown of foliage leaves is renewed 

 by the unfolding of the large terminal bud. 



The species are all dioecious, the male and female flowers 



