LEPIDODENDRON. 



67 



below the point where they pass off into the leaf. At first their direction is almost 

 parallel with the cylinders, slightly inclining outwards ; they then incline more outwards ; 

 and as they approach the circumference of the stem, they resume their nearly ascending 

 direction for some distance, until they finally pass out to the leaves which they support. 

 Each bundle consists of scalariform vessels, very much finer than those of the woody 

 cylinder, surrounded by elongated cells, like those of the outer zones, and probably still 

 further enclosed by a delicate parenchyma, whicli disappeared before it could be fossilized. 

 The only evidence I have of the existence of this cellular tissue is that the bundles never 

 fill the cavities in the parenchyma of the stem through which they pass. The bun- 

 dles terminate in the points seen on the areolas of the stem, which are the scars of the 

 leaves. 



"The woody cylinder is of diff'erent thicknesses in different stems, and appears to have 

 increased with the growth of the tree. There is, however, no indication of interruption 

 in the growth, or of seasonal layers. Yet it cannot be conceived that the whole vascular 

 cylinder arose and was developed at the same time. It is very probable that the zone of 

 slender and consequently of rarely preserved cellular tissue, which surrounds the woody 

 cylinder, was analogous in its functions to the cambium layer of phanerogamous stems, 

 like the similar layers in recent Lijcopodiacea, described by Spring in his ' Monographic 

 de la Eamille des Lycopodiacees' (p. 294). 



" If we separate the different structures we have described in the axis into two series, 

 the one series axial, and the other epidermal, we shall have the axis composed of scalariform 

 utricles, the woody cylinder and the vascular bundles passing to the leaves belonging to 

 the first series, and the two external zones of the vascular tissue to the second. The 

 inner zone of cellular tissue, like the cambium layer, was most probably common to both 

 series, the cells of the outer circumference being developed into the parenchyma of 

 the epidermal series, while the vessels of the woody axis were produced from the cells of 

 the inner series." 



Here come in the paragraphs on the Strobihis of Lepidodendron and its stigmarioid 

 roots, quoted by me at length in pages 37 and 38 of Part II of this Monograph. Then 

 the author proceeds, " In speculating upon the conditions under which the forests of 

 Lepidodendron flourished, it is most important to observe whatever is peculiar in those 

 organs by which the plants were connected with the physical conditions around them. 

 Geologists have too much overlooked such considerations in their deductions as to the 

 physical phenomena of a period from the plants and animals that then existed. They 

 have often taken for granted that the known conditions of the living species of a genus 

 are true also of the fossil members of the same genus. In the want of other evidence 

 such an assumption may be cautiously employed ; but, unless its true value be accurately 

 estimated, the greatest errors may arise, as they have in the past. For example, the 

 systematic position of the Mephas primic/enius having been clearly established, the inference 

 was thought legitimate that, as the modern representatives of the genus were confined to 



