420 STUDIES IN FOSSIL BOTANY 



petal, while a small portion is developed centrifugally. 

 The phloem-zone is surrounded by a pericycle several 

 cells thick. 



The general structure of the stele is thus very 

 closely similar to that in Heterangium. The leaf- 

 traces, one of which passes out to each leaf, are not 

 alike in detail, and we must not push the analogy too 

 far. The anatomical agreement between Gleichenia 

 and Heterangium is, however, quite near enough to 

 prove that the stem-structure of the latter genus is 

 essentially that of a protostelic Fern, from which it 

 differs in scarcely anything but the presence of 

 secondary tissues, and the mode of pitting of the 

 tracheides ; the latter character, however, is common 

 to many of the Botryopterideae. 



If Heterangium, and especially H. Grievii, stood 

 alone, we should have little reason for regarding it as 

 anything more than a Fern, which had acquired the 

 power of secondary growth, just as the fossil Lycopods 

 acquired it without thereby becoming anything more 

 than Lycopods. But even within the same genus, 

 when we consider H. tiliaeoides, it is impossible not 

 to be struck by the resemblance of the secondary 

 tissues to those of Cycads, and by the distinctness of 

 the peripheral strands of the wood, recalling the mesarch 

 xylem of Cycadean foliar bundles. 



When we come to Lyginodendron, which we know 

 to have been a Seed-plant, the vegetative anatomy still 

 shows many points of agreement with Ferns, though 

 the Cycadean characters are more evident. The 

 primary bundles are here isolated from one another, 

 and disposed around a wide pith. The whole organi- 



