734 FOSSIL PLANTS OF THE CAEBONIFEKOUS SYSTEM. 



Lepidodendrons. Professor W. C. Williamson, who has examined 

 with great care the fossil carboniferous Flora, has detected in many 

 of the plants an apparent exogenous mode of formation in the stem. 

 According to him the stem of a Lepidodendron consists of a central 

 medullary axis containing scalariform vessels and cells. It is sur- 

 rounded by a narrow ring of a similar nature, but arranged in 

 vertical laminae radiating from within outwards. The laminae are 

 separated by cells, arranged like the medullary rays of an Exogen. 

 From the outer cylinder vessels go to the leaves. Outside the woody 

 zone there is a cortical portion, formed by parenchymatous and 

 prosenchymatous cells. The whole is covered by an epidermis, con- 

 sisting of a cellular layer, then a bast layer, and finally a superficial 

 cellular layer. The outer epidermal covering is often removed, and is 

 sometimes converted into coal. The stem increases in a more or less 

 exogenous manner, whUe the cortical portion retains aU the characters 

 of Lepidodendroid plants. Williamson thinks that there is an evident 

 transition from the vascular Cryptogams to the Gymnospermous Exo- 

 gens, and that they cannot be separated. There are some difficulties 

 in deciding on the exogenous development of a fossil stem. To deter- 

 mine this properly, we require to demonstrate the existence of Cambium 

 cells, and it is not easy to do so in fossUised plants. Care is also re- 

 quired in pronouncing on the mode of development, seeing that the 

 thick stems of cellular plants, such as seaweeds, sometimes exhibit 

 concentric circles, and the same thing occurs in the succulent roots of 

 some annual and biennial plants. The beautiful microscopical pre- 

 parations made by Professor Williamson certainly show in many 

 instances marked zones with rays. Full details of his researches are 

 given in the Transactions of the Eoyal Society of London, illustrated 

 by excellent plates. 



The slender terminal branches of Lepidodendron are known under 

 the name of Lycopodites. Ulodendron (ilAjj, wood, and dhdgov, 

 tree) is a genus nearly allied to Lepidodendrons. Hugh Miller 

 states that Ulodendron minus, found in ferruginous shale in the 

 Water of Leith, near Oolinton, exhibits beautiful sculptured scars, 

 ranged rectilinearly along the stem. The surface is covered with 

 small, sharply-relieved obovate scales, most of thiem furnished with 

 an apparent midrib, and with their edges slightly turned up. The 

 circular or oval scars of this genus are probably impressions made 

 by a rectilinear range of aerial roots placed on either side. When 

 decorticated, the stem is mottled over with minute dottings, arranged 

 in a quincuncial manner, and its oval scars are devoid of the ordinary 

 sculpturings. Bothrodendron (^od^og, a pit or depression) is a decor- 

 ticated condition of Ulodendron. Halonia appears also to be a species 

 of Lepidodendron. The scars of Ulodendron, and the tubercles of 

 Halonia, are probably the remains of special organs, such as cones. 



