May 22, 1896.] 



SGIENGE. 



755 



school still range them by the side of the 

 Gymnosperms. 



As representing the latest stage in the 

 progress of knowledge concerning exogen- 

 ous development in the Paleozoic Lyco- 

 pods, as well as expressing the views of the 

 foremost authorities in Paleozoic plant his- 

 tology in both the Brongniartian and the 

 English schools, I venture to summarize, 

 in brief, without pretense of adding any- 

 thing original to the subject myself, the 

 contents of two lately published papers. 



The first, by the late Prof. W. C. Wil- 

 liamson, of Owens College, England, his 

 last independent publication, I believe, is 

 entitled, On the light thrown upon the question 

 of the Gfroivth and Development of the Carbon- 

 iferous Arborescent Lepidodendra by a study of 

 the details of their Organization.^ 



At the outset it may be well, and of in- 

 terest to the reader, to briefly review the gen- 

 eral structural characters of the Lepidoden- 

 dron type, in describing which I shall quote 

 in part from Prof. "Williamson's own publi- 

 cations: "In the youngest Lepidodendroid 

 twigs the conspicuous central tissue is a 

 small vascular bundle known as the pri- 

 mary xylem strand. It extends, under 

 varied modifications of form and size, from 

 near the apex of the youngest twig to the 

 base of the oldest stem. In its downward 

 course it gives off a large number of small 

 vascular bundles, known as leaf traces, 

 each one of which passes outwards to a leaf, 

 supplying it with its vascular tissues. In 

 many cases we discover a few cells in the 

 center of its component tracheids, which, 

 on passing downward towards the lower 

 members of the tree, enlarge into a more 

 or less conspicuous medulla. In a few cases 

 the smaller shoots exhibit no traces of these 

 cells, which are only discoverable in 

 branches of somewhat larger size; but in 

 all, the larger the twig, the larger, also, is 



*Mem. Proo. Manchester Lit. and Phil. Soc, 

 1894-95, pp. 31-65, 1895. 



the central cellular tissue in varying de- 

 grees and in different types. This is a true 

 medulla, which generally exhibits its maxi- 

 mum diameter only at the base of the oldest 

 stems." 



In the closest external contact to this 

 pi-imary xylem system is a second vascular 

 zone, the ' secondary xylem,' which is de- 

 veloped from a peripheral cambium layer 

 much like the growth of ordinary trees. 

 This secondary xylem is composed of ver- 

 tically prolonged radiating vascular 1am- 

 infe, which are separated by intervening 

 medullary rays. These two systems form 

 the ' stele, ' and the Carboniferous Lycopods 

 are 'monostelic' The remaining external 

 zones of tissue constitute the leaf bearing 

 cortex. " In its youngest state this tissue 

 consists almost wholly either of rounded 

 cells, parenchyma, or vertically elongated 

 ones with pointed ends, prosenchyma." At 

 a later period of growth, varying in differ- 

 ent types, a thin meristemic zone appears in 

 the outermost parenchyma of the cortex. A 

 ring of its rounded cells, as seen in trans- 

 verse section, undergoes divisions, the more 

 internal developing into prosenchymatous 

 ones to form a layer of periderm. This per- 

 iderm constantly thickens by similarly pro- 

 duced exterior additions so long as the 

 plant lives, constituting the great bulk of 

 the tree trunks, which may attain a diam- 

 eter of four feet or more. The outermost 

 cells resulting from the above-described 

 meristemic action experience a succession 

 of similar metamorphoses, always preserv- 

 ing a thin layer of parenchyma between the 

 surface of the periderm and the bases of the 

 leaves. The leaves, which are variable in 

 form and size, are attached by rhomboidal 

 bases to the bolsters or leaf cushions, which, 

 though square and hardly larger than the 

 leaf base when young, continue to grow 

 after the true leaf falls off, and their dia- 

 mond-shaped, often fusiform protuberant 

 bolsters, arranged in quincunx, form the 



