XV] 



LEPIDODENDRON 



101 



Lepidodendron leaf-cushion has been worked out in detail by 

 Weiss who shows that, at least in some species, the two arms 

 do not bend downwards as shown in the diagram, fig. 144, B, 

 but pursue a straight gradually ascending course as seen in 

 fig. 145, A. Just below the leaf-scar region of the cushion each 

 arm comes into association with a group of lacunar, aerenchy- 

 matous tissue, such as occurs in the roots of certain Mangroove 



B 



pL- 



,^-d 



--It 



— e 



Pig. 144. Lepidodendron Veltheimianum Stemb. 



A. Leaf-cushion and leaf -scar seen in surface- view at a ; on the rest 

 of the specimen a slightly lower surface is exposed. (After 

 Stur.) 



B. Diagrammatic longitudinal section to explain the differences 

 between its two surfaces a and 6 shown in fig. A. 



The shaded portion c represents the rock matrix, the surfaces 

 ab, ed, mark the outer and inner edge of the outer portion of 

 the bark of the Lepidodendron stem. 

 It, leaf-trace ; p, p', pariohnos. 



plants, and it is this aerenchyma which is exposed on the two 

 oval depressions below the leaf-scar. The structure of this 

 aerenchyma is shown in fig. 145, B ; it consists in this species 

 {L. Hickil Wats.) of stellate cells which would constitute an 

 efficient aerating system. Probably, as Weiss suggests, these 

 patches of aerenchyma were originally covered by an epidermis 

 provided with stomata, and it is owing to the destruction of this 

 superficial layer that the two oval scars often form a prominent 



