66 LYCOPODIALES [CH. 



arranged in parallel series on each side of the median line of the 

 two or three furrows. The three arms of the triangular stele of 

 /. hystrix and the two narrow ends of the long axis of the stele 

 of /. lacustris, which in transverse section has the form of a 

 flattened ellipse, are built up of successive root-bases. A root 

 of Isoetes (fig. 133, G) possesses one vascular bundle, x, with a 

 single strand of protoxylem, px, thus agreeing in its monarch 

 structure with the root-bundle in Selaginella and many species 

 of Lycopodium. The cortical region of the root consists of a few 

 layers of outer cortex succeeded by a large space, formed by 

 the breaking down of the inner cortical tissue, into which the 

 vascular bundle projects (fig. 133, F). The peculiarity of the 

 roots in having a hollow cortex and an eccentric vascular bundle 

 was noticed by Von MohP. In the monarch bundles, as in the 

 fistular cortex and dichotomous branching, the roots of Isoetes 

 present a striking resemblance to the slender rootlets of the 

 Palaeozoic Stigmaria (see Page 246). The longitudinal section 

 through the base of a root of Isoetes lacustris shown in fig. 133, F, 

 affords a further illustration of certain features common to the 

 fossil and recent types. 



FOSSIL LYCOPODIALES. 



Isoetaceae 



The geological history of this division of the Pteridophyta is 

 exceedingly meagre, a fact all the more regrettable as it is by 

 no means improbable that in the surviving genus Isoetes we 

 have an isolated type possibly of considerable antiquity and 

 closely akin to such extinct genera as Pleuromeia and 

 Sigillaria. If Saporta's Lower Cretaceous species Isoetes 

 Choffati'^, or more appropriately Isoetites Choffati, is correctly 

 determined, it is the oldest fossil member of the family and 

 indeed the most satisfactory among the more than doubtful 

 species described as extinct forms of Isoetes. 



1 Von Mobl (40). 



2 Sapoita (94) p. 134, Pis. xxiv. xxv. xxvii. 



