402 



VASCULAR CRYPTOGAMS. 



having the characteristic folding, but in the Equisetaceae it is the last layer but 

 one of the cortical tissue which presents this appearance, whilst the innermost 

 layer which directly abuts upon the axial cylinder seems to supply the place of 

 the pericambium which does not exist in the roots of these plants. This innermost 

 layer differs from the pericambium of the roots of other Vascular Cryptogams in 

 that the lateral roots take origin from it, so that here also, as in all other Vascular 

 Cryptogams, the lateral roots arise from the innermost layer of the cortical tissue. 

 As a pericambium is wanting here, the commencing roots arise in immediate 

 proximity to the external vessels of the axial cylinder. The cells, each one of 

 which gives rise to a lateral root, are formed in strictly acropetal succession in 

 the innermost cortical layer on the outer side of the primary xylem-vessels ^ 



The Sporangia of Equisetaceae are outgrowths of peculiarly metamorphosed 

 leaves which are generally formed in numerous whorls at the summit of ordinary 



<-' .3z / r t 



Fig. 284. — Diagram of the succession of cell-divisions in the apex of the root of Equisetum hiemale (after Nageli and 

 Leitgeb), (this diagram will serve also in the main for Ferns and for Marsih'a). A longitudinal section ; B transverse section at 

 the lower end oi A ; h h h the primary walls, s s s the sextant walls of the segments, indicated in A by the figs. I— XVI, 

 k I m n p the layers of the root-cap, all the further divisions being omitted ; c c in the interior of the root indicates the walls by 

 which the rudimentary fibro-vascular cylinder (procambium) is divided from the cortex of the root, e the boundary-wall between 

 the epidermis o and the cortex (epidermal wall), r r boundary-wall between the outer and inner cortex (cortical wall), i, 2, 3, the 

 successive tangential walls by which the inner cortex is divided into several layers, the radial divisions being omitted. 



shoots or of those specially destined for this purpose. Above the last sterile leaf- 

 sheath of the fertile axis an imperfectly developed leaf-sheath is first of all produced 

 (Fig. 285,^), a structure corresponding in some degree to the bracts of Phanerogams. 

 The development of this structure is sometimes more, sometimes less leaf-like ; foliar 

 girdles are formed above it in acropetal succession beneath the growing end of 

 the shoot, projecting however but slightly, as in the ordinary formation of leaves 

 of Equisetum. A large number of protuberances project from each of these girdles, 

 corresponding to the teeth of the ordinary leaf-sheaths ; and thus several whorls of 

 hemispherical projections are formed lying closely one over another, which, in- 

 creasing more rapidly in size at their outer part, press against one another, and 

 thus become hexagonal, the successive whorls alternating; while the basal (inner) 



^ [See Van Tieghem, La Racine, Paris 187 1.] 



