420 VASCULAR CRYPTOGAMS. 



whole of the under side of the leaf. The chlorophyll in the cells of the leaf of Sela- 

 ginella often forms only a few — sometimes only one or two — masses variable in form ; 

 the margin of the leaf consists, in this genus, of only a single row of cells, which, as in 

 Mosses, develope in the form of teeth or hairs. 



To this brief description must be added a few further words with respect to Isoetes. 

 The short stem of the mature plant contains an axial woody body which can scarcely be 

 termed a bundle, consisting of short roundish vascular cells united loosely, and with 

 spiral or reticulated thickening-bands. P'rom these the fibro-vascular bundles proceed, 

 one into each of the very numerous leaves (Fig. 305, p. 407) and into the roots. Not- 

 withstanding H. V. Mohl's accurate description (/. c), Hofmeister's statements, and my 

 own researches, it is still impossible to compare this peculiar fibro-vascular body morpho- 

 logically with the bundles of Lycopodium and Selaginella. But in opposition to the view 

 that the layer of tissue which surrounds it is a cambium analogous to that of Dicoty- 

 ledons and Conifers, it may be objected that this thick layer of meristem which invests 

 the fibro-vascular body produces on the outside only parenchymatous fundamental tissue, 

 by which the outer masses of parenchyma that annually decay away and become brown 

 are replaced. In this respect this tissue is comparable rather to the thickening-ring of 

 Dracaena, which also forms on the outside new cortical parenchyma and on the inside 

 new fibro-vascular bundles. The true cambium of Dicotyledons, on the other hand, 

 produces fibro-vascular structures in both directions, on the outside phloem, on the 

 inside xylem. But the stem of Isoetes probably possesses no proper fibro-vascular 

 bundle at all ; it would appear rather, from the position of the vessels, that the axial 

 fibro-vascular body consists only of the lower (inner) commencements of the foliar 

 bundles, which are here densely crowded. In the same manner the basal disc-like 

 woody body may consist only of the densely crowded commencements of the radical 

 bundles. If this view is correct, the class of Lycopodiacese presents two extremes, one 

 in Psilotum, where the foliar development is small, and where there are, according to 

 Nageli, no foliar bundles, but the elongated stem forms a fibro-vascular bundle belong- 

 ing to it only ; the other in Isoetes, where the short stem possesses no cauline fibro- 

 vascular bundle, and only the strongly developed leaves have one each. The structure 

 of the leaves of Isoetes varies according as the species grow submerged in water, in 

 marshes, or on dry ground. In the first case they are long and conical, penetrated by 

 four air-cavities divided by septa into channels, with a weak fibro-vascular bundle in 

 the axis of the organ, and the epidermis destitute of stomata : in the second case they 

 are similar, but provided with stomata and hypodermal vascular bundles ; in the third 

 case the epidermis is also provided with stomata, and the basal portions of the dead 

 leaves (phyllopodes) form a firm black coat of mail round the stem. 



[Professor W. C. Williamson has contributed the following note on the Carboniferous Lyco- 

 podiaceae : — ' The large and varied group of the Lycopodiaceous plants of the Coal Measures exhibits 

 so many modifications that it is difficult to give a brief statement of their characteristic features. But 

 so far as the Lepidodendroid and Sigillarian forms are concerned, our British forms all exhibit one 

 type of internal organisation. In the very young state each twig has a central bundle of scalariform 

 vessels surrounded by a 'bark,' which usually exhibits an inner parenchymatous layer surrounded by 

 a more prosenchymatous one, which is again invested by a second but more unequal parenchyma. 

 Bundles of vessels given off by the central vascular axis proceed to each of the leaves. As the twig 

 enlarges the central axis invariably expands into a vascular cylinder, its interior becoming occu- 

 pied by a cellular parenchyma of large size, and which now occupies the position and exhibits the 

 appearance of a true medulla. The parenchyma of the leaves appears to be an extension of the 

 outermost parenchyma of the bark. The above remarks appear to represent the common history of 

 all the Lepidodendroid plants up to a certain stage of their growth. Beyond this stage their histories 

 vary somewhat in the different groups. In some forms, e.g. those to which the Haloniae belong, 

 the branches attain considerable dimensions without undergoing any great change in their internal 

 organisation ; but in others a new development of vascular tissue invests the central cylinder at a 

 period which seems to have varied in different species. This new growth takes place in successive 



