STRUCTURE AND AFFINITIES OF A LEPIDODENDROID STEM. 927 



recent plants must in the Palaeozoic types have been carried out by tissues of another 

 kind and of a more primitive character. This question will be dealt with in a forth- 

 coming account of some exceedingly well-preserved examples of Lepidophloios fuli- 

 ginosus (Will.).* It is a remarkable fact tha,t there is no band of tissue which can be 

 spoken of as secondary phloem at all comparable in extent to the secondary xylem. 

 The absence of any well-defined initial meristem layer or typical cambium is another 

 point of general interest. The meristem zone in the Dalmeny stem is separated from 

 the newest tracheids by a comparatively broad band of cells, the elements of which 

 appear to assume more gradually the form of xylem tracheids than is the case with most 

 plants which possess the power of secondary growth ; we should speak of a cambial 

 zone rather than a cambium layer as constituting the generative region from which the 

 wood was supplied with new elements. 



The thick band of secondary cortex, described as phelloderm, is a character common 

 to the Lepidodendra and Sigillarise generally. The meristem-zone, which arises just in- 

 ternal to the leaf-cushions, at an early stage in the growth of these plants gives rise to 

 little or no tissue on the outside, but on the other hand produces internally a consider- 

 able thickness of radially disposed elongated elements, and some of these cells undergo 

 further divisions to form strands of secretory tissue ; from analogy with other plants 

 one may speak of the internal secondary cortex as phelloderm, a tissue with a fairly 

 well-developed aerating system of intercellular spaces. The cork of recent plants, in 

 the sense of an impermeable tissue developed on the outside of the phellogen, is barely 

 represented. It may be noted that Worsdell has described a thick band of phelloderm 

 in the stems of Cycas media, R. Br.,t and Macrozamia.\ 



When we consider the small diameter of the woody central cylinder, as shown in 

 PL I. fig. 1, in comparison with the size of the stem as a whole, the usefulness of the 

 wood as a mechanical support to enable the stem to resist lateral strains would seem to 

 have been extremely small. May we not regard the thick phelloderm of the fossil stem 

 as a tissue of w T hich one of the functions was mechanical ; a strengthening baud at the 

 periphery of the stem corresponding to the hypodermal stereome or to the peripheral 

 region of secondary wood in recent plants ? § 



The exact significance of secretory products in recent plants is by no means well 

 understood, and seeing that we cannot determine the nature of the secretions in fossil 

 tissues, we are not in a position to do more than hazard a guess as to the function of 

 the glandular tissue. The abundance of secretory canals in so many of the Palaeozoic 

 genera is an interesting fact, and one which, with further data derived from physio- 

 logical anatomical considerations, may help us to learn something as to the conditions 

 under which these plants grew. 



In the outermost stelar region of Lepidodendra the large secretory sacs form a 

 striking feature, and the close connection between strands of this secretory zone and the 

 outgoing leaf-trace reminds one of the large secretory strands which are met with in the 



• [Seward (99).] t Worsdell (98), p. 443. i Ibid. (96), p. 615. § Cf. Haberlandt (96), p. 163. 



