XIV] ISOETES 63 



the laminae of the leaves have been removed from the summit 

 affords an example of a species with two furrows. The 

 drawing shows the widely gaping sides of the broad furrow 

 with circular root-scars and a few simple and dichotomously 

 branched roots. A short thick column of parenchymatous 

 tissue projects from a slightly eccentric position on the base of 

 the stem. 



The primary vascular cylinder' consists of numerous spiral, 

 annular or reticulate tracheids (fig. 133, A, x), which are either 

 isodiametric or longer in a horizontal than in a vertical 

 direction, associated with parenchyma. Lower in the stem 

 crushed and disorganised xylem elements are scattered through 

 a still living trabecular network of parenchymatous tissue. 

 From the axial cylinder numerous leaf-traces (fig. 133, A, It) 

 radiate outwards, at first in a horizontal direction and then 

 gradually ascending towards the leaves. The vascular cylinder 

 is of the type known as cauline ; that is, some of the xylem is 

 distinct in origin from that which consists solely of the lower 

 ends of leaf-traces. As in Lycopodium the development of the 

 metaxylem is centripetal. 



Von MohP, and a few years later Hofmeister^, were the first 

 botanists to give a satisfactory account of the anatomy of 

 Isoetes, but it is only recently* that fresh light has been thrown 

 upon the structural features of the genus the interest of which 

 is enhanced by the many points of resemblance between the 

 recent type and the Palaeozoic Lepidodendreae. A striking 

 anatomical feature is the power of the stem to produce secondary 

 vascular and non-vascular tissue ; the genus is also charac- 

 terised by the early appearance of secondary meristematic 

 activity which renders it practically impossible to draw any 

 distinct line between primary and secondary growth. A 

 cylinder of thin- walled tissue (fig. 133, A, a) surrounds 

 the primary central cylinder and in this a cambial zone, c, is 

 recognised even close to the stem-apex ; this zone of dividing 

 cells is separated from the xylem by a few layers of rectangular 

 cells to which the term prismatic zone has been applied. 



1 See von Mohl (40); Farmer (90). - Von Mohl (40). 



s Hofmeister (62). " Farmer (90) ; Scott and Hill (00). 



