3/6 VASCULAR CRYPTOGAMS. 



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 tv^^o 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-walled 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 Thsue 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 JE. Telmateia. The green colouring of the other shoots 

 is caused by layers consisting of from i to 3 strata of parenchyma containing chlorophyll 

 (the cells lying transversely). This green tissue lies especially beneath the furrows, cor- 

 responding to the stomata, and forms on a transverse section ribbon-shaped masses 

 concave outwardly ; in the slender leafy branches, where the ridges sometimes cause 

 the transverse section to have a stellate outline {e.g. E. arvense') the tissue containing 

 chlorophyll is in excess. The vallecular canals, which correspond to the furrows, arise 

 in the fundamental tissue by separation and partially by rupture of the cells ; they may 

 be absent from the slender leafy branches. 



The Fibro-'vascular Bundles are arranged, in a transverse section of the internodes, 

 as in Dicotyledons, in a circle, each corresponding to a ridge of the surface, between 

 the cortical canals but somewhat nearer the centre. In the axis of the sporangiferous 

 stems, where the diaphragms are wanting, they run in the same manner, and bend out 

 singly into the pedicels of the peltate scales (as in the sheath-teeth). The bundles of 

 a shoot are all parallel to one another ; each bundle is the result of the coalescence 

 of two portions ; one of these belongs to the leaf-sheath and developes in the median 

 line of one of its teeth from below upwards ; the other portion developes in the internode 

 itself from above downwards. At the angle where the two portions meet, the form- 

 ation of tissue begins in both, and thence advances in opposite directions; the lower 

 end of each bundle unites by two lateral commissures with the two next alternate 

 bundles of the next lower internode ; and the Equisetaceae have therefore only ' common ' 

 bundles. In transverse sections these bundles resemble the fibro-vascular ones of Mono- 

 cotyledons, especially of Grasses ; the first-formed annular, spiral, or reticulated vessels 

 belonging to the inner side, together with the thin-walled cells which separate them, are 

 subsequently destroyed, and a canal (carinal) remains in their place traversing the whole 

 length of the fibro-vascular bundle on its inner side. Right and left of this lie on the 

 outside a few not very broad vessels thickened reticulately ; external to the canal lies 

 the phloem-part of the bundle, formed of a few wide sieve-tubes and narrow cambiform 

 cells, and at the circumference of a few thick-walled narrow bast-like cells. These are 

 all enveloped by a prosenchymatous tissue. A vascular-bundle-sheath, as it is termed, 

 either surrounds each bundle or (varying with the species) runs continuously outside 

 the circle of all the bundles ; sometimes (as in E. hyemale, &c.), a similar layer of 

 tissue is found on the inner side of the circle of bundles as well. 



[Professor W. C. Williamson is led by a study of the internal organisation of Calamites and 

 Calamodendra ^ to the conclusion that in England at least we have but one group of these fossil 

 plants. When young their vascular zone, separating a medullary from a cortical parenchyma, 



^ [On Fossil Equisetaceae, see Williamson, Mem. Lit, Phil. Soc. Manch. 3rd ser, vol. IV, 

 pp. 155-183 ; Ditto, Trans. Roy. Soc. vol. CLXI, pp. 477-510. — Coemans, Joum. Bot. 1869, pp. 337- 

 340. — Dawson, Ann. Nat. Hist. 4th ser. vol. IV, pp. 272-273. — Grand'Eury, Ann. Nat. Hist. 4th ser. 

 vol. IV, pp. 124-128; Compt. Rend. vol. LXVIII. — M^Nab, Joum. of Bot. 1873, pp. 72-80. — Ed.] 



