12 WATSON, Two Species of Lepidodendron Harconrtii. 



The parichnos communicates with the surface before 

 reaching the leaf scar by the two lateral scars. 



There are two shallow depressions, on the lower 

 surface of the cushion, which pass in laterally until they 

 are underneath the parichnos strands. 



The parichnos broadens out to reach these pits, and 

 this broadening is of rather finer parenchyma than the 

 normal parichnos, and also seems to have fewer air 

 spaces. {Fig- 5, Plate II.) 



The normal structure of the parichnos is that it is a 

 strand of parenchyma, the cells of which are rather small, 

 iso-diametric, and more or less spherical, so as to leave 

 considerable intercellular spaces. 



This description applies to the majority of the sections, 

 but in the series in the Williamson collection, No. 380 B — L, 

 in several places this pit is seen to be filled up with a 

 typically aerenchymatous tissue. (See Fig. 11, Plate III.) 

 This tissue is composed of star-shaped cells which leave 

 very marked air spaces, and agree exactly with those 

 described by Professor Weiss (:07) in some isolated sections 

 of Lepidodendron. 



In this last case, however, the pit is empty, and the 

 aerenchymatous tissue opens into it, being separated from 

 its cavity by an epidermis. 



This difference suggests that the lateral scars seen in 

 impressions are really of two types, one in which they 

 represent actual pits, during the life of the plants, and 

 the other in which there are really no pits but a patch of 

 spongy and easily decaying tissue. 



The pit in L. vasculare, described by Hovelacque 

 ('92), seems to differ from those in Professor Weiss's 

 sections, in that it has no obvious connection with the 

 parichnos, which is separated from it by many apparently 

 rather thick-walled cells. 



