EQUISETACE.E. 375 



ascending axis have only to extend and the slender lateral branches to unfold, as may be 

 seen with especial ease in E. Tdmateia. All the more important cell-formations and the 

 processes of morphological differentiation thus take place underground ; the aerial un- 

 folding has for its main purpose only the dispersion of the spores and assimilation in the 

 leafy shoots, by the exposure of the cortex, which contains chlorophyll, to light. The 

 rapid growth of the upright stems in the spring is brought about especially by the simple 

 elongation of the internodal cells already formed, although permanent intercalary growth 

 of the internodes sometimes also takes place, and especially at their base within the 

 sheaths. The tissues often remain there for a long time in the young state, and in E. 

 hyemale the internodes, still short and lighter in colour after passing through the winter, 

 push themselves out of their leaf-sheaths ; the shorter they were before the winter, the 

 more they elongate afterwards. 



Special Organs for Fegetatiir Propagation, like those of Mosses, are not found in the 

 Equisetaccce any more than in Ferns ; but every part of the rhizome, and the under- 

 ground nodes of ascending stems, are adapted for the production of new stems. In 

 some species some of the underground shoots swell up into ovoid (£. ar'vense) or pear- 

 shaped {E. Telmateia) tubers about the size of a hazel-nut; Duval -Jouve states that 

 these occur also in E. paliistre, syh'aticum, and littorale, but in other species {E. pratense, 

 limosion, ramosissitnum, 'variegatu>7i, and hyemale) they have not yet been observed. The 

 tubers are produced by the rapid increase in thickness of an internode at the end of 

 which is situated the terminal bud ; this may repeatedly form tuberous internodes so that 

 the tubers become moniliform, or they may dcvelope simply as a rhizome, or sometimes 

 a central internode of a rhizome is developed in a tuberous manner. The parenchyma 

 of these tubers is filled with starch and other food-materials ; they may apparently long 

 remain dormant and form new stems under favourable circumstances. 



Among the Forms of Tissue of the Equisetaceae the epidermal system and the funda- 

 mental tissue are in particular developed in a great variety of ways. The fibro-vascular 

 bundles, which in Ferns are so thick and so highly organised, especially in their xylem- 

 portion, appear to be less developed in the Equisetacea? ; they are slender, the lignification 

 of the xylcm-portion very slight (as in many water and marsh plants) ; the firmness of 

 their structure is chiefly due to the epidermal system with its highly developed epidermis, 

 and to the hypodermal fibro-vascular bundles. What follows has special reference to the 

 internodes ; the leaf-sheaths are usually similarly constituted in their lower and central 

 parts ; at the teeth the tissue is simpler and more uniform. 



The Epidermal Cells are mostly elongated in the direction of the axis, and are 

 arranged in longitudinal rows separated by transverse or slightly oblique walls; the 

 boundary-walls of the adjoining cells are often undulating. The epidermis of the 

 underground internodes is almost always destitute of stomata, and consists of cells with 

 either thick or thin M-alls, usually brown, which, in some species, as E. Telmateia and 

 arirnse, develope into delicate root-hairs. The epidermis of the deciduous sporangi- 

 ferous stems of the species just named is similar to that of the rhizome and without 

 stomata; and the same is the case with the upright colourless sterile stem of E. Tel- 

 mateia. In all the aerial internodes which contain chlorophyll, the leaf-sheaths, and 

 the outer surface of the peltate scales, the epidermis possesses numerous stomata which 

 always lie in the channels, never on the ridges, and are arranged in longitudinal rows 

 either single or lying close to one another. On the ridges the epidermal cells are 

 long, in the channels between the stomata shorter. All the cells, even those of the 

 stomata, have their outer walls strongly silicified, and exhibit very often on their outer 

 surface protuberances of various forms, which are also and indeed peculiarly strongly 

 silicified. These protuberances resemble fine granules, bosses, rosettes, rings, transverse 

 bands, teeth, and spines; on the guard-cells they usually occur in the form of ridges, 

 running at right angles to the orifice. The guard-cells are generally partially overreached 

 by the neighbouring epidermal cells. The mature stoma appears to be formed of two 

 pairs of guard-cells lying one over another; Strasburger asserts that these four cells 



