20G MR R. J. D. GRAHAM ON 



pericyclic stereoms, linked radially by an incomplete mesocortical web (Plate II. fig. 10). 

 In all the specimens examined the contours of the hypodermal groups of fibres were 

 fairly uniform, being a flange or triangle with a longer or shorter portion projecting 

 centripetal ly. The mesocortical and the pericyclic groups of fibres vary considerably, 

 in some cases inversely. The mesocortical web consists of isolated fibres or groups of 

 fibres, usually numerically less than the hypodermal or pericyclic groups. In some 

 cases the mesocortical groups are poorly developed, and in a few cases are almost 

 suppressed ; when this occurs the hypodermal is usually well developed. The single 

 mesocortical fibres resemble those of Welwitschia in having calcium oxalate deposited 

 in their external membranes (Plate II., fig. 10). This is also the case in the hypodermal 

 fibres of E. Helvetica. The pericyclic stereom appears as a series of hard bast crescents 

 or aggregates associated with the primary vascular bundles, one large or a few smaller 

 ones to each (Plate II. , fig. 11). Between adjacent crescents occasionally a series of 

 isolated fibres occur. 



A perimedullary stereom in the form of a discontinuous ring of sclerenchyma occurs 

 in some species (Plate II., fig. 12) ; in others it is represented by a few isolated fibres or 

 nests at intervals, while it may be absent altogether. 



Vascular System. — The vascular system resembles that found in Equisetum, with 

 these differences, that in this case the leaf-trace is paired, that it is continued in the 

 cauline part of its course through two internodes, and that in certain species an 

 accessory bundle accompanies each leaf-trace through part of its course, while some 

 species show a fusion of the leaf-traces in part of their course. With these exceptions 

 the bundles in each describe a similar course, alternating in each internode and forming 

 vascular networks at the node. The leaves, reduced to mere scales, are situated in twos 

 or threes at each node. In the former case the arrangement is opposite and decussate, 

 in the latter the whorls alternate. When there are two leaves at the node, the normal 

 shoot shows in cross section a system of six, eight, or ten collateral endarch bundles 

 surrounding a large pith. The structure of the primary bundle is as follows 

 (Strasburger, 5). The whole is surrounded by a parenchyma sheath, some of the cells 

 of which may contain chlorophyll. The primary phloem consists of cambiform 

 parenchyma elements and narrow sieve-tubes with oblique plates. The primary xylem 

 consists of spiral elements and bordered pitted elements with parenchyma packing 

 tissue. The bordered pitted elements include both tracheids and vasa, the latter 

 having their end walls steeply inclined and perforated by one or two rows of slightly 

 bordered holes (Von Mohl, 6). Between the bundles medullary rays occur, but soon 

 the bundles are linked up by the completion of a cambium, ring. Projecting from the 

 xylem elements of the system in some species isodiametric lignified cells with reticulate 

 thickening occur, and these at an early stage link up the adjacent bundles (Plate II., 

 fig. 11). These cells are the first products of the activity of the cambium. Extending 

 from the xylem to the medullary rays, as they do, they facilitate a rapid lateral 

 distribution of crude sap to the chlorenchyma. 



