EQUISETINEAE. 



271 



of ridges running at right angles to the orifice. The guard-cells are usually partially 

 overlapped by the subsidiary epidermal cells. The stoma when fully developed appears 

 to be formed of two pairs of guard-cells lying one over the other ; these four cells, 

 according to Strasburger, proceed from a single epidermal cell and lie at first side 

 by side in a transverse row ; eventually the two inner cells, the true guard-cells, are 

 pressed inwards by the two outer cells which grow more vigorously, and are overlapped 

 by them. Beneath the epidermis both of the rhizomes and upright stems, as well as of the 

 leafy shoots (with the exception of the deciduous fertile stems), strands or layers of firm 

 thick-walled cells, hypodermal tissue, are commonly found in the Equisetaceae ; they 

 form in the rhizomes a continuous stratum of many layers of brown-walled sclerenchyma ; 

 in the aerial internodes they are colourless and most strongly developed in the projecting 

 ridges. 



The fundamental tissue of the internodes consists chiefly of a colourless thin-walled 

 parenchyma, which is found 'only in the rhizomes, the deciduous fertile stems and the 

 colourless sterile stems of E. Telmateja ; the green colour of the other shoots is due to 



FIG. 225. Stem of Equisetum hiemale, and a stoma with its environment. A view of the inner surface ; the pair 

 of guard-cells girthed at the side by the edge of the superposed pair of subsidiary cells. B transverse section of the stem 

 passing through the middle of a stoma, which lies in a depression of the surface ; the narrow orifice is bounded by the 

 two flat guard-cells and the subsidiary cells which surround them ; the cells of the epidermis and of the sclerenchyma 

 beneath it have numerous pit-canals. C silicious residuum of a small piece of epidermis with a stoma after maceration 

 in Schulze's mixture and subsequent ignition, seen from the outside. The curvilinear figures are the outlines of the pro- 

 minences of the outer surface. From De Bary, Vergl. Anatomic. 



a 1-3 layered stratum of parenchyma containing chlorophyll, in which the cells lie trans- 

 versely. This green tissue lies chiefly beneath the furrows, being associated with the 

 stomata on their upper surface ; it is ribbon-like in outline as seen in a transverse section 

 and concave outwardly, and preponderates in the slender leafy branches, where the 

 ridges sometimes give a stellate form to the transverse section, as in E. aruense. The 

 lacunae (vallecular canals) which lie on the same radii as the furrows are formed in the 

 fundamental tissue by the separation of the cells from one another, sometimes by their 

 rupture ; they are sometimes wanting in the slender leafy branches. 



A transverse section of an internode shows the vascular bundles arranged in a circle, 

 as in the Dicotyledons, each on the same radius with a ridge on the surface and between 

 the cortical lacunae or nearer the axis. In the axis of the sporangiferous spikes, where 

 there are no diaphragms, their course is the same, and they bend outwards into the 

 stalks of the peltate disks, one into each, as they do into the foliar teeth. The bundles 

 in a shoot are all parallel with one another ; each bundle is formed by the coalescence 

 of two arms, one of which belongs to the leaf-sheath and is formed in the median line 



