418 PART III. THE CLASSIFICATION OF PLANTS. 



lamina. Thus the sporophyll is peltate. It bears on its inner (dorsal) surface, 

 a small number (5-10) of sporangia arranged round the leaf-stalk. 



The sporangia are somewhat elongated in fcim, 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 (not tetrahedral) 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 are 

 nearly spherical when ripe. Each spore has two coats, exine and intine, and 

 originally a perinium is present. The perinium, as it developes, becomes 

 irregularly thickened in such a way that, when the thin portions are destroyed, 

 the thickened portions remain as four filaments, the elaters, all attached at one 

 point only to the spore. These elaters are very hygroscopic. When the air is 

 dry they expand, and stand out stiffly from the spore ; when moistened, they 

 suddenly roll up spirally round the spore. The spores become entangled by 

 their elaters, so that when set free from the sporangium a number of the 

 spores fall to the ground, and germinate near together. 



The roots are all adventitious, though a short-lived primary root is developed. 

 Tliey are developed at the nodes of the rhizome. 



General Histology. A striking feature in the anatomy of the stem is the pre- 

 sence of large, mainly lysigenous, air-cavities : thus, in some species, the rhizome 

 has a large central cavity in each internode (E. silvaticum [Fig. 273 C, a], ar- 

 vense, maximum) ; a similar cavity is present in the internodes of the aerial 

 shoots of nearly all the species (Fig. 273 A,a)\ the central cavities of successive 

 internodes are shut off from each other by diaphragms at the nodes (Fig. 271 n) : 

 a series of similar cavities occurs always in the cortex, alternating with the vas- 

 cular bundles internally and with the surface-ribs externally, hence termed 

 vallecular cavities (Fig. 273 I) ; finally, in connexion with each vascular 

 bundle, there is a large cavity, the carinal cavity (Fig. 273 c), extending, like 

 the others, from one node to another. 



Tbe growing-point of the stern, and of its branches, is apical, and has a tetra- 

 hedral apical cell (Figs. 112, 113). The stem is, except at first, schizostelic 

 (see p. 152) ; the schizosteles may be either distinct (rhizomes and aerial shoots 

 of E. limosum and litorale [Fig. 2735] ; rhizomes of E. hiemale, trachyodo)i, 

 and ramosissimum) ; or they may fuse (gamodesmic, see p. 170), so that the 

 endodermis becomes continuous, forming either only a well-marked external 

 layer (rhizomes and aerial shoots of E. arvense, maximum, pratense, scirpoide*, 

 palustre [Fig. 274 A] ; aerial shoots of E. silvaticum), or well-marked external 

 and internal layers (rhizome of E. silvaticum [Fig. 2T4 C] ; rhizome and aerial 

 shoot of E. variegat uin ; aerial shoots of E. hiemale, tracJnjodon, ramosissimum). 



Each schizostele contains a single bundle which is collateral, closed and 

 common, with very rudimentary xylem consisting of the few annular vessels of 

 the protoxylem and of two small groups of scalariform tracheids. 



In the aerial shoots (except specialised fertile shoots of E. arvense, etc.) there 

 is a considerable development of assitnilatory tissue in the cortex, strands of 

 this tissue corresponding in position with the furrows on the surface in which 

 the stomata are developed ; whilst opposite the ridges on the surface there are 

 cortical strands of sclerenchyma. The development of assimilatory tissue in 



