362 ME. ABTHUE JOHN MASLEN ON THE 



Another peculiarity in the vascular cylinder of this type is seen in the abundance 

 of short tracheides ("barred cells") in the peripheral region. PL XXXVI. fig. 2 is a 

 transverse section showing the vascular cylinder, inner cortex, and emerging leaf-trace 

 bundles, some of the short tracheides being shown at s.t. PI. XXXVl. fig. 3 is a longi- 

 tudinal section of the xylem cylinder ; it is not, however, truly radial, and so does not 

 pass through the pith. The larger tracheides x. exhibit scalariform thickening, and are 

 quite similar to those shown on PL XXXVIII. fig. 24 from Lepidostrohus oldhamius (|3). 

 At the periphery can be seen some of the characteristic short tracheides. Two of these 

 short tracheides from the same section (C.N. 1613) are shown more highly magnified 

 on PL XXXVI. fig. 4. 



Some of the smaller peripheral xylem elements appear to show spiral thickening, and 

 doubtless here, as in other Lepidodendra, the development was centripetal, the proto- 

 xylem occupying the periphery of the wood. 



At first thought it might appear that in the presence of these short tracheides we have 

 something similar to the arrangement seen in the axis of Lepidodendron selaginoides, and so 

 characteristic a feature of that type of stem *. There is, however, the striking difference 

 that, whereas in L. selaginoides the barred cells are found intermingled with ordinary 

 parenchymatous cells about the centre of the stele, in Lepidostrohus oldhamius (a) these 

 short tracheides are found only at the periphery oi the xylera, among the smaller elements 

 which give rise to the leaf-trace bundles, so that there is no real resemblance between 

 the two forms, excepting in so far as they both possess similar short barred elements. 



Another feature which is beautifully preserved in this form is the presence of a com- 

 paratively thick zone of parenchymatous tissue completely investing the xylem cylinder. 

 This tissue is best seen in the longitudinal sections (see PL XXXVI. figs. 3 & 5). In both 

 these figures it will be seen that the tissue in question, p., consists of fairly thin- 

 waUed, generally flat-ended cells, showing an arrangement in vertical series. The cells 

 are but little elongated in the longitudinal direction, and the vertical sections show no 

 trace of anything here besides ordinary parenchyma ; there is no indication of sieve-tubes 

 or special phloem elements of any kind. Moreover, there is some evidence that these ceUs 

 passed at the periphery into a more delicate tissue with longer cells (see PL XXXVL fig. 3), 

 but the latter tissue lias almost entirely disappeared from around the central axis of the cone. 



As will be shown later, there is abundant evidence in the leaf -trace (sporophyll) bundles 

 of the presence of a tissue between the xylem and phloem, which is here fairly well 

 preserved ; and there can be little doubt that around the central axis the true phloem is 

 represented by a portion of the empty space within the inner cortical zone, and that there 

 was a distinct parenchymatous tissue between the xylem and phloem. 



The same tissue can also be seen in the transverse sections (see PL XXXVI. fig. 2, p.). 



Through this tissue the leaf-trace bundles pass, diverging only very gradually from the 

 xylem cylinder, as shown on PL XXXVI. fig. 5, l.t'., in longitudinal, and PL XXXVL 

 fig. 2, It., in transverse section. See also the diagram, PL XXXVII. fig. 22. 



Cortex. — As Prof. Bower has shownf, there is in Lepidodendron a diff<erentiation of 



* See figures in Williamson, "Organization," Part II., Phil. Trans. 1872, and Part XI., PhiL Trans. 1881. 

 t Ann. Bot. vol. vii. p. 343. 



