Xl] DEFINITION. 391 



linear segments. The leaves of successive whorls are super- 

 posed. 



The strobili are long and narrow in form, having a length in 

 some cases of 12 cm., and a diameter of 12 mm. ; they occur as 

 shortly stalked lateral branches, or terminate long leaf-bearing 

 shoots. The axis of the cone bears whorls of numerous linear 

 lanceolate bracts fused basally into a coherent funnel-shaped 

 disc, bearing on its upper surface sporangiophores and sporangia. 



The strobili are usually isosporous, but possibly heterosporous 

 in some forms. 



The stem is monostelic, with a triarch or hexarch triangular 

 strand of centripetally developed primary xylem, consisting of 

 reticulate, scalariform and spiral tracheae ; the protoxylem 

 elements being situated at the blunt corners of the xylem- 

 strand. Foliar bundles are given off, either singly or in pairs, 

 from each angle of the central primary strand. The secondary 

 xylem consists of radially disposed reticulate or scalariform 

 tracheae, developed from a cambium-layer. The phloem is 

 made up of thin-walled elements, including sieve-tubes and 

 parenchyma. Both xylem and phloem include secondary 

 medullary rays of parenchymatous cells. The cortex consists 

 in part of fairly thick-walled elements; in older stems the 

 greater part of the cortical region is cut off by the development 

 of deep-seated layers of periderm. 



The roots are apparently diarch in structure, with a lacunar 

 and smooth cortex. 



The branch of Sphenophyllum emarginatum Brongn. given in 

 fig. 109 shows the characteristic appearance of the genus as 

 represented by this well-known species which Brongniart 

 figured in 1822. The Indian species shown in fig. Ill illus- 

 trates the occurrence of unequal leaves in the same whorl, and 

 in fig. 110, B, we have a form of verticil in which the leaves 

 are deeply divided into filiform segments. A larger-leaved 

 form is represented by S. Thoni, Mahr. (fig. 110, i4), a species 

 occasionally met with in Permian rocks. 



No specimens of Sphenophyllum have so far been found 

 attached to a thick stem ; they always occur as slender shoots, 



