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, scalai-iform 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 r etic ulate 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 

 medullaiy 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 Spkenophylliim 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, A), 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. 



