CONIFERALES (PIXACKAE) 241 



The sporophylls follow the leaf arrangment; for example, they 

 are spiral in the Abietineae, and cyclic in the Cupressineae. They 

 are exceedingly variable in form, nearly every genus having a charac- 

 teristic microsporophyll. In almost every case there is an evident 

 differentiation into a stalklikc base and an expanded terminal region 

 (representing a lamina) bearing the sporangia. In some cases, as in 

 Widdringtonia (159, 176) among the Cupressineae, the lamina spreads 

 as a peltate expansion, the several sporangia arising from the stalk 

 and girdling it. In other cases, as in the Araucarineae, the lamina 

 is represented by a knoblike enlargement, beneath one side of which 

 the free sporangia hang, suggesting the stamen of Ginkgo, and also 

 the Crossotheca ("epaulet") type among the Cycadofilicales (fig. 265). 

 There is every gradation between this more or less peltate type and 

 the bladelike expansion of the sporangium-bearing region in the plane 

 of the stalk. The bladelike tip shows all degrees of development, 

 from conspicuous (as in Cedrus and Cunninghamia) to much reduced 

 (as in Pinus), and with its abaxial sporangia suggests the most primi- 

 tive type of sporophyll, namely that of the Filicales, which was con- 

 tinued among the Cycadofilicales. So far as known, no living conifer 

 has the type of stamen known to belong to some of the Cordaitales, 

 namely a stamen with terminal and erect sporangia; but doubtless 

 these two other types also belong to that great paleozoic group. 



The number of sporangia is quite variable, being always two among 

 the Abietineae, two to five (perhaps more) among the Taxodineae, two 

 to six among the Cupressineae, and five to eight or more among the 

 Araucarineae. It is to be remembered that these numbers apply only 

 to living representatives of these tribes, and that they doubtless varied 

 much more widely among their predecessors. 



The development of the microsporangium is of the usual euspo- 

 rangiate type, that is, the archesporium is a hypodermal layer of cells, 

 varying in number, which divides periclinally to form the primary 

 wall layer (outside) and the primary sporogenous layer (inside). The 

 primary wall layer, by successive periclinal divisions, produces several 

 wall layers (usually four or five in the Abietineae, three in Taxodium, 

 and two or three in the Cupressineae), the innermost one of which 

 forms a ]mrt of the conspicuous tapetum, and the outermost one some- 

 times developing as an endothecial layer (76). While in general 



