338 



LYCOPODIALES 



differentiation of' the primary wood is nearly simultaneous over its whole 

 area, but with indications of centripetal succession. The cambial activity 

 starts early, being continuous from that of the primary meristem. As a 

 rule the same cambium is active throughout, producing secondary ground 

 tissue, wood, and phloem on its inner side, and cortical parenchyma 

 only towards its exterior; but other arrangements are found, while in 

 some cases a second cambial activity may arise inside or without the 

 first. The adjoining diagram, quoted from Scott and Hill (Fig. 177), 

 shows the relation of the primary and secondary tissues usual in /. hystrix^ 

 and it will be noted that the secondary phloem is internal to the secondary 

 xylem ; the cambium lies outside the latter in direct contiguity with the 



secondary cortex, which arises externally 

 from it. The stele which is cylindrical 

 above becomes in /. hystrix triquetrous 

 below, in /. lacustris it is usually flattened 

 bilaterally : the change of form is a 

 secondary consequence of the abutment 

 of the numerous, successively formed 

 root-bases upon it, and does not affect 

 the general comparisons. Scott and Hill 

 conclude that the anatomy of the stem 

 with its solid stele, from which the 

 densely crowded small and simple leaf- 

 traces pass off, is just what might be 

 expected in a stunted Lycopod, while the 

 anomalous character of the secondary 

 thickening in Isoetes agrees in some 

 measure with that in certain fossil Lyco- 

 pods. Scott 1 has remarked on the stem 

 of Lepidodendron fuliginosiim as having 

 an anomalous cambium producing a 

 good deal of secondary parenchyma, among which there are scattered 

 groups of wood ; and he regards this species as exhibiting a primitive 

 and rudimentary form of secondary growth. It seems to offer a distinct 

 analogy with Isoetes. On the other hand, the slight cambial increase 

 discovered in Selaginella spinulosa by Bruchmann affords some link as 

 regards secondary thickening, though a feeble one, with a living Lycopod. 

 The general result of this anatomical examination and comparison of 

 Isoetes is accordingly to strengthen its position among the Lycopodiales, 

 and to show that its primary vascular arrangement corresponds in 

 essentials to the type as exemplified in living species of Lycopodium, 

 but much abbreviated, and with the xylem reduced in accordance 

 with the aquatic or amphibious habit prevalent in the genus. It also 

 appears that the secondary development, though showing fluctuating 



1 Studies, p. 143. 



FIG T 



Diagrammatic transverse section of the upper 



dary phloem has developed ; ,r 2 , that in which 

 secondary xylem has developed ; r< = cambium ; 

 c<i secondary cortex; It leaf-traces. X42. 

 (After Scott and Hill.) 



