248 PTERIDOPHYTA. [CH. 



the tubers and the tips of the terminal tubers bear small leaf- 

 sheaths. Branched roots are also given off from the upper node 

 of the erect shoot. 



Near the surface of the ground the buds on the rhizome 

 nodes develope into green erect shoots. The shoot axis is 

 marked out into long internodes separated by nodes bearing 

 the leaf-sheaths. The surface of each interned e is traversed 

 by regular and more or less prominent longitudinal ridges and 

 grooves; each ridge marking the position of an internal longitu- 

 dinal vascular strand. In the axil of each leaf, that is in the 

 axil of each portion of a leaf-sheath corresponding to a marginal 

 uni-nerved tooth, there is produced a lateral bud which may 

 either remain dormant or break through the leaf-sheath and 

 emerge as a lateral branch. At the base of each branch an 

 adventitious root may be formed from a cell immediately below 

 the first leaf-sheath, but in aerial shoots the roots usually 

 remain undeveloped. The lateral branches repeat on a smaller 

 scale the general features of the main axis. In some species, 

 the shoots are unbranched, and in others the slender branches 

 arise in crowded whorls from each node. Leaves, roots and 

 branches are given off in whorls, and the whorls from each node 

 alternate with those from the node next above and next below. 



In some species of JSquisetiim the aerial stem terminates in 

 a conical group of sporophylls, while in others the strobilus is 

 formed at the apex of a pale-coloured fertile shoot, which never 

 attains any considerable length and dies down early in the 

 season of growth (fig. 52, A). Below the terminal cone or 

 strobilus there occur one or two modified leaf-sheaths. Such 

 a ring of incompletely developed leaves intervening between 

 the cone of sporangiophores and the normal leaves, is known as 

 the annulus. The annulus is seen in fig. 52, A, immediately 

 below the lowest whorl of sporophylls ; it has the form of a low 

 sheath with a ragged margin. In the region of the cone the 

 internodes remain shorter, and the whorls of appendages, known 

 as sporophylls or sporangiophores, have the form of stalked 

 structures terminating distally in a hexagonal peltate disc, 

 which bears on its inner face a ring of five to ten oval sporangia 

 (fig. 52, B). Each sporangium contains numerous spores which 



