ON COAL. 157 



out from the softer cellular tissue. Here we o'bserve no distinct pitli ; on 

 distinct bark separable from the wood ; the wood not collected into a 

 distinct zone; not arranged into concentric layers, nor divided by me- 

 dullary rays. The exogenous plan of structure includes the Dicoty- 

 ledons and the pine and cycas families ; while endogen may be con- 

 sidered synonymous with monocotyledon. 



In the vascular Cryptogams the woody and vascular tissue is still 

 differently arranged. The stem of a club-moss, for instance, consists 

 of a mass of cellular tissue inclosed in a rind of the same tissue more 

 condensed, with a single central thread of vascular tissue. Sometimes 

 there seems to be in the centre of this something like a very imperfect 

 pith. The cellular Cryptogams, as their name indicates^ consist 

 entirely of cellular tissue. 



It will be observed that, in the general structure and mode of 

 growth, the family of pines (Gymnosperms) is allied to the highest 

 order of plants, viz: the Dicotyledons, while in its reproduction it is 

 below the Monocotyledons. This latter position is beyond doubt the 

 true one ; and a more attentive examination of the wood of pine in 

 comparison with that of Dicotyledons will confirm us in this view. 

 As this is a very important point, and as much false theorizing on 

 the subject of the plants of the coal has been the result of a miscon- 

 ception of the true position of conifers, I will dwell a little more 

 minutely than I should have otherwise considered it necessary to do. 



The wood oi' Dicotyledons consists of two dis- Fis. 20. 



tmct tissues, viz: the woody tissue jiroper and the 

 vascular tissue. The woody tissue proper is com- 

 posed of elongated cells, too small to be distin- 

 guished by the naked eye, while the vascular tissue 

 is composed of very much larger cells or tubes. 

 The visible pores in such wood as oak^ chestnut, 

 vine, &c., belong to this tissue. Fig. 20 repre- 

 sents cross section of two wooden wedges, with 

 their medullary rays. The comparative size of 

 the wood cells and the vessels is well shown. The 

 difference is often much greater than in the figure. 

 In pine wood, on the contrary, there is no distinction of woody and 

 vascular tissue; but the so-called wood consists entirely of an open, 

 thin-walled tissue, intermediate in every respect between the vas- 

 cular and the woody layer and thinner walled than the true woody, 

 but smaller than the true vascular. This is shown in the cross 

 section, (fig. 21.) On a longitudinal section, (fig. 22,) the cells 

 of pine wood are marked by large disc-like elliptical plates, which 

 are entirely characteristic of this family. The smallest fragment 

 is sufficient to distinguish it with the utmost certainty. 



Now, if we trace the development of the tissues, either by passing 

 from the lowest to the highest plants, or from the earliest embryonic 

 to the mature condition of one of the higher plants, we shall find that 

 all the different kinds of tissue are modifications of the cellular ; that 

 there is a more and more complete differentiation of form and special- 



