162 



LECTURES 



A cross and longitudinal section of the Lepidodendron shows 

 similar but still less highly organized structure, (figs. 29 and 30.) 



Fig. 29. 



Fig. 30. 



The vascular sheath is still smaller, extremely thin, forming on 

 cross section an exceedingly narrow zone. It is moreover not separated 

 into concentric rings nor divided by medullary rays. The cellular 

 tissue both within and without the sheath is very open and loose. 

 The rind {d) consists of similar cellular tissue, but more condensed, 

 and there seems to be no line of demarcation, but a gradual transition; 

 in other words, there is apparently no true bark. Here, also, we find 

 long slender bundles of vascular tissue (spiral vessels) connecting the 

 leaves with the central sheath. Microscopic examination of the vas- 

 cular sheath shows no sign of woody tissue. 



Calamites we know much less about, but it would seem that in them 

 there is a still greater predominance of cellular tissue, if, indeed, they 

 possessed any vascular tissue at all. They are often found pressed 

 perfectly flat, indicating that they were either hollow, or more probably 

 consisted of a simple rind of condensed cellular tissue, inclosing looser 

 tissue of the same kind. Of this plant, however, we know too little to 

 draw any conclusion as to its affinities. 



Now, if we examine by sections a common Lycopodmm, or club-moss, 

 we find an internal structure closely resembling what we have found 

 in Sigillaria and Lepidodendron. Externally a thin but tough rind, 

 or epidermis of condensed cellular tissue, inclosing a mass of very loose 

 cellular tissue, through the centre of which runs a slender thread of 

 vascular tissue, sending off in every direction still slenderer threads of 

 the same to the crowded leaves. Upon longitudinal section the vascu- 

 lar tissue is found to be chiefly spiral ducts. The principal difference 

 between this structure and that of the Lepidodendron is that the latter 

 has a more perfect pith, and in this respect seems to be allied to the 

 higher order of plants. But I am convinced, from personal examina- 

 tion of the Lycopodium, that its vascular thread was the outline of both 

 pith and medullary rays. I call more particular attention to this 

 observation, because^ as far as I know, it is new, and as it seems to me 

 calculated to throw much light on the affinities of coal plants. 



This very remarkable structure, viz : the existence of a slender 

 central thread of vascular tissue in the midst of a large mass of very 

 loose cellular, does not exist, I believe, among existing plants in the 



