382 



PART IV. CLASSIFICATION. 



shoot throws out branches, and thus comes to be a sterile shoot. In some 

 species the sub-aerial shoots are generally unbranched (e.g. E. hiemale 

 trachyodon, variegatuni). 



The leaves are either cataphylls or sporophylls. The cataphylls, repre- 

 senting the foliage-leaves, are borne in whorls at the nodes, having a 

 common attachment, so that they form a leaf-sheaf at each node. They 

 are small and brown in colour. 



The sporophylls, like the 

 cataphylls, are developed 

 in whorls, but owing to 

 the fact that the inter- 

 nodes between the whorls 

 do not elongate, the sporo- 

 phylls are aggregated into 

 a cone-like flower situated 

 terminally on the shoot 

 (Fig. 228), or less common- 

 ly (e.g. E. palustre) on some 

 of the lateral branches. 

 The leaf-sheath below the 

 cone, marking off the re- 

 productive from the vege- 

 tative region of the shoot, 

 is peculiar, being gener- 

 ally very much reduced, 

 and is termed the ring 

 (see p. 57). 



Each sporophyll has a 

 small hexagonal lamina 

 which is inserted on the 

 axis of the cone by a short 

 stalk attached to the 

 centre of the inner surface 

 of the lamina. Thus the 

 sporophyll is peltate. It 

 bears on its inner (dorsal) 

 surface a small number 

 (5-10) of sporangia ar- 

 ranged round the leaf- 

 stalk. 



The sporangia are some- 

 what elongated in form, 



and are sessile. The wall of the sporangium consists of a single layer 

 of cells with spiral thickening. Dehiscence takes place longitudinally 

 on the side facing the leaf-stalk. The archesporium is usually a single 

 cell from which are derived the mother-cells of the spores, each of which 

 give rise to four spores. 



The spores, which are all of one kind, are developed tetrahedrally, but 



lv 



FIG. 228. A Upper portion of a fertile branch of 

 Equisetum palustre. v Leaf-sheaths, below which the 

 branches (r) spring; w the uppermost sterile sheath 

 (ring); a the flower; s the peltate sporophylls. B 

 Transverse section of the stem ( x6) : c central cavity ; 

 s the vascular bundles arranged in a circle, each having 

 a carinal cavity, fc ; / the vallecular cavities ; r the 

 ridges. C Sporophyll with sporangia ( x 10) : t the 

 stalk ; sp the sporangia. I) Diagram of the course 

 taken by the vascular bundles where two internodes 

 meet ; i i the internodes ; fc the node. 



