SIGILLARIA 225 



however, to be clear that the secondary xylem and the 

 transfusion-tracheides are continuous, one with another ; 

 the relation's of the tissues require further investigation. 



The mesophyll is lacunar in the neighbourhood of 

 the stomatiferous furrows ; elsewhere it is described 

 as consisting of transversely elongated elements, which 

 may have enabled the leaf to roll up its lamina, and 

 •thus .diminish transpiration. There is a band of 

 sclerenchyma below the epidermis, except in the 

 stomatiferous furrows. The stomata are of the ordinary 

 kind ; they are accompanied on the epidermis of the 

 furrows by multicellular hairs (Fig. 92, C, g). The 

 whole arrangement of the tissues of the leaf is sugges- 

 tive of a plant occasionally exposed to drought, but 

 we must remember that the plants of salt marshes 

 assume in many respects a xerophytic habit. 



It will be seen that the leaves of the species of 

 Sigillaria in question agree nearly, though not exactly, 

 in structure with those of Lepidodendron, as described 

 above in Chapter V. The differences between them 

 are such as are often found even among species of the 

 same genus ; the resemblances, taken in conjunction 

 with the other characters, are undoubtedly indicative of 

 near affinity. 



It appears probable that the leaves of Sigillaria 

 were separated from the stem by the formation of a 

 definite abscissile tissue. 



So far we have dealt exclusively with the structure 

 of the Sub-Sigillarian group. Our knowledge of the 

 anatomy of the Eu-Sigillariae has until recently been 

 extremely meagre, but is now much augmented. A 

 fragment of stem with Favularian surface was described 



15 



