270 



THIRD GROUP. VASCULAR CRYPTOGAMS. 



the size of a hazel-nut, which are ovoid in form in E. arvense, pear-shaped in E. Tel- 

 mateja ; Duval-Jouve states that these tuberous stems occur also in E. palustre, E. syl- 

 vaticum, and E. littorale, but have not yet been observed in E. pratense, E. limosum, 

 E. ramosissimum, E. hiemale, or E. variegatum ; they are due to considerable increase 

 , in thickness of an internode, and have a bud at their extremity, which may produce a 

 succession of tuber-like internodes like a string of beads or may develope as a simple 

 rhizome ; sometimes an internode in the middle of a rhizome developes into a tuber. 

 The parenchyma of these tubers is filled with starch and other food-material, and may, 

 it would seem, remain dormant for a long time, and then under favourable conditions 

 give rise to new stems. 



Of the forms of tissue in the 

 Equisetaceae the dermal system 

 and the fundamental tissue are the 

 two which show the greatest variety 

 of structure; the vascular, bundles, 

 which are so stout in the Ferns and 

 so highly organised especially in 

 their xylem, are less favoured in 

 the Equisetaceae ; there they are 

 slender and their xylem is very 

 slightly lignified, as is the case also 

 in many water and marsh plants ; 

 the firmness of their structure is 

 chiefly due to the dermal system 

 with its highly developed epidermis 

 and to the hypodermal bundles of 

 fibres. The following remarks refer 

 chiefly to the internodes ; the lower 

 and middle portions of the leaf- 

 sheaths are much alike in structure, 

 while the tissues are different and 

 more simple in the teeth. 



The cells of the epidermis are 

 usually elongated in the direction 

 of the axis, and disposed in longi- 

 tudinal rows in which the adjoining 

 cells have tranverse or slightly 

 oblique walls, and the walls between 

 two cells are often undulated. The 

 epidermis in underground -inter- 

 nodes is almost always free from 

 stomata and is composed of cells 

 which have thick or thin, usually brown walls, and in many species, as E. Tebnateja and 

 E. arvense, develope into delicate root-hairs. The epidermis of the deciduous fertile stems 

 in the above species and in the sterile erect colourless stem of E. Telmateja is like 

 that of the rhizomes, and has no stomata. In all other aerial internodes with chloro- 

 phyll in their tissues, in the leaf-sheaths and in the outer surface of the peltate disks, 

 the epidermis forms numerous stomata, all lying in the furrows never on the ridges, 

 and arranged in single longitudinal rows or in several lying close to one another. The 

 epidermal cells are longer on the ridges, shorter in the furrows between the stomata. 

 The outer walls of all cells, even, those of the stomata, are strongly silicified ; their 

 surfaces often show protuberances of various forms, granules or bosses, rosettes, rings, 

 lobes, transverse bands, teeth or spikes, which are still more strongly silicified ; pro- 

 minences of this kind on the guard-cells of the stomata (Fig. 225) usually take the form 



FIG. 224. Part of a transverse section through a full-grown internode 

 of Equisetum palustre; u endodermis, * axile air-canal, at x remains of 

 the walls of shrivelled pith-cells, in the middle a vascular bundle 

 without distinct sheath surrounded by parenchyma. At the interior 

 margin of the vascular portion is a wide intercellular passage in which the 

 letters.?, r, t are inscribed ; / a ring, still adhering to the membrane, from 

 the wall of a primary tracheide, the greater part of which is destroyed, 

 r persistent annular tracheides, g groups of the last formed annular and 

 reticulated tracheides which are also persistent, distinguished from the 

 surrounding parts by the shading of the walls, s the sieve-tube portion 

 (phloem) in which the broad lumina belong to the sieve-tubes, the nar- 

 rower and sometimes granularly dotted to the cambiform cells. The 

 doubly-contoured bands on the outer edge of the phloem, inside the cell- 

 layer following u, indicate the collapsed protophloem. From De Bary, 

 Vergl. Anatomic. 



