On the Structure, of Lepidodendron selaginoides. 103 



bark, but I believe it has no relation whatever to the cortical 

 system, and it probably corresponds to the sheath of the 

 vascular bundle (Gefassblindelscheide of German authors) 

 (PL II., Figs. 1, 2, and 5, c, and PL Il.a, Fig. 2, c).* 



Immediately outside of the last-mentioned structure is a zone 

 of large, firm-celled parenchyma, the cells of which become 

 much smaller towards its outer portion. When this portion of 

 the plant is examined in longitudinal section, the cells towards 

 its inner portion are of an oval form, those towards its periphery 

 are somewhat elongated but cannot be described as prosenchy- 

 matous (PL IL, Figs. 1, 2, and 3, d, and PL Il.a, Fig. 2, d). 

 These pass gradually into the more delicate and narrower 

 cells of a layer which presents the characters of a meristem 

 (PL IL, Figs. 1, 2, and 3,/, and PL Il.a, Fig. 2,/). 



The cortex in this plant consists, in my opinion, of an in- 

 ternal portion — the aforesaid zone, and of an external part in 

 connection with the leaf bases, with a meristem between them 

 (PL IL, Figs. 1 and 2, d, g). The cells towards the periphery 

 of the internal zone pass into (or more correctly seem to pass out 

 of) a layer of cells, presenting all the characters of a meristem, 

 or formative zone, out of which additions were no doubt 

 made from time to time, to the outer portion of the inner 

 bark (PL IL, Figs. 1, 2, and 3,/, and PL Il.a, Fig. 2,/). 



The inner, larger celled portion of the bark, as I have 

 defined it, has been called the middle or parenchymatous 

 part of the bark, and its outer portion, the outer or prosenchy- 

 matous part of the bark, distinctions which are certainly not 

 shown in the specimen exhibited to-night. 



The meristem layer varies from five to ten cells in breadth. 

 In transverse section its cells appear quadrangular, flattened 

 from without inwards, and much smaller than the outer cells of 



* The term " vascular bundle sheath" is here used in a somewhat wider 

 sense than that usually employed. As employed in the present paper, it em- 

 braces, in addition to the "sheath" proper, all the tissue lyin<5 between it 

 and the vascular bundle, so it will probably include the soft bast, which there 

 is every reason to believe would be situated in this region. 



In some recent Lycopodiums, and even among individuals of the same 

 species, the prominence of the "bundle sheath" varies considerably — so 

 much so, that, in some cases, it seems hardly distinguishable from the soft 

 bast within it. 



