146 



MORPHOLOGY 



(figs. 339, 340). The outer walls of the epidermal cells are so impreg- 

 nated by a deposit of silica as to give the characteristic rough feeling to 

 the stem. The stem is fluted, and within the ridges strands of fibrous 

 cells are developed; while in the furrows the chlorophyll tissue reaches 

 the epidermis. It follows that the stomata (see p. 250) are in the furrows 

 (usually along the slopes) rather than on the ridges. Deeper within 

 the cortex a zone of large air passages occurs, each one lying beneath a 

 furrow. The central cylinder or stele is remarkably reduced, the vas- 

 cular bundles being very feebly developed. They are arranged (usually 

 one beneath each ridge) so as to outline a hollow cylinder enclosing a 

 pith, which disappears early; but instead of being concentric bundles, 

 characteristic of most pteridophytes, they are collateral; that is, the 

 xylem and phloem strands lie side by side on the same radius, with 

 the xylem towards the center of the stem. In fact, however, in Equi- 

 setum the xylem is hardly at all developed, its position being occupied 

 by a small air passage (fig. 340). 



Sporangium. The sporangia occur in a very distinct strobilus 

 (fig. 332). The structure usually called a sporophyll has a stalklike 

 base and a peltate top, beneath which five to ten sporangia are borne 

 (figs. 334, 335)- 



To understand this structure it is necessary to be familiar with certain of the 

 extinct Equisetales. A series can be arranged, beginning with Sphenophyllum, 

 passing on to Calamites (an extinct group of Equisetales), and ending with Equise- 

 tum, which indicates that in the strobilus of Equisetum the sporophylls have been 



suppressed and that the structures bearing spo- 

 rangia are sporangiophores. In Calamites the 

 strobilus is made up of alternating sets of sporo- 

 phylls and sporangiophores, and the latter are 

 just such structures as appear in the strobilus of 

 Equisetum, without the alternating sets of spo- 

 rophylls. In Selaginella there are sporophylls 

 that do not bear sporangia ; and in Equisetum 

 there is apparently a strobilus that does not 

 consist of sporophylls. 



342 



FlGS. 341, 342. Sporangium 

 of Equisetum: 341, early stage, 



The sporangium arises from a single 



three cells having arisen from the /~ i 11 . r 



.. . superficial cell, and not from a transverse 



superficial initial, the innermost 



(shaded) being sporogenous, the row as among Lycopodiales. There is 



two outer being wall cells; the usual periclinal wall, setting apart an 



342 , further development of wall u j f . s 



layers and sporogenous tissue. r c 



After GOEBEL. cell, as in all eusporangiates (fig. 341). 



