4o6 
VASCULAR CRYPTOGAMS. 
some species some of the underground shoots swell up into ovoid (E. ar'vense) or pear- 
shaped {E . Telmateia) tubers about the size of a hazel-nut; Duval-Jouve states that 
these occur also in E. palustre, syl'vaticuniy and littorale, but in other species [E. pratense, 
limosum, ramosissimum, variegatum, and hiemale) 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 develope 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 Equisetaceae ; they are slender, the lignification 
of the xylem-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 fibres. 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 somewhat different. 
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 walls, usually brown, which, in some species, as E. Telmateia and 
arnjense, 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. On all the aerial internodes which contain chlorophyll, on the leaf-sheaths and 
on 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 covered 
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 
arise from one epidermal cell, and lie at first side by side at the same level. Only 
at a later period the two inner ones (the true guard-cells) become pressed inwards and 
overreached by the two outer ones which grow more rapidly. Bundles or layers of firm 
thick-walled cells (Hypodermal Tissue) are of common occurrence beneath the epi- 
dermis of rhizomes, of upright stems, and of their leafy shoots (with the exception 
of the deciduous sporangiferous stems). In the rhizomes they form a continuous 
stratum of brown-wailed sclerenchyma consisting of several layers ; in the aerial inter- 
nodes they are colourless and are developed with especial prominence in the projecting 
ridges. 
The Fundamental tissue of the internodes consists in the main of a colourless thin- 
walled parenchyma occurring only in the rhizomes, the deciduous sporangiferous stems, 
and the colourless sterile axes of E. Telmateia. The green colouring of the other shoots 
1 [For further details on this subject, see De Bary, Vergleichende Anatomie der Vegetations- 
organe der Phanerogamen und Farne, 1877.] 
