390 SYDNEY H. VINES. 



consisting, not as in Pteris, of a single layer of cells, but of 

 several layers, and the external sheath also consists of 

 numerous layers. 



In Selaginella (ineequalifolia), in the stem of which, as in 

 that of Pteris, the bundles are isolated, each bundle is sur- 

 rounded by a cavity, its connection with the ground tissue 

 being maintained by bridles of cells, which reach from the 

 one to the other. The bundle is bounded towards the air 

 cavity by a definite layer of cells, which is apparently the 

 bundle sheath. E-ussow^ regards the lacunar tissue as repre- 

 senting this structure, but the previous view seems to be 

 probably the correct one, when a comparison is made of the 

 bundle of Selaginella with that of the fern and ofLycopo- 

 dium. Within the bundle sheath we find a phloem sheath, 

 consisting, as in Lycopodium, of several layers of cells. The 

 external sheath may, perhaps, be represented here by the 

 lacunar tissue and by the Avell-defined layer of cells which 

 bounds the air cavity on the side of the ground tissue. 



In the stems,rhizomes, and roots of many Monocotyledons an 

 evident bundle sheath is to be found (PL XXVII, fig. 1). It is 

 especially constant in the Cyperacese and Juncacese, and often 

 occurs in Graminese. We find no structures in the bundles 

 of the Phanerogamia which correspond to the phloem sheath^ 

 in the vascular Cryptogams, but the external sheath is often 

 well developed. In fig. 1 we see a dense mass of cells (bast 

 fibres) investing nearly the whole of the circumference of a 

 fibro-vascular bundle, and evidently corresponding to the 

 structure referred to in the Fern. In those Monocotyledons 

 which do not possess a well-marked bundle sheath it seems 

 difficult to decide whether these bast fibres belong to an 

 external sheath, or whether they are elements of the 

 fibro-vascular tissue. To this difficulty we shall hereafter 

 refer. 



A bundle sheath occurs in the bundles of the roots of the 

 great majority of the Dicotyledons, and of all Gyninospernis, 

 and of the stems of many Dicotyledons (Campanulacese, 

 Primulaceae, &c.), which invests the whole of the vascular 

 bundles.^ 



In some Dicotyledons each bundle has its own sheath, as 

 in Eranthis hyemalis, Ranunculus Imgua and Jlammula 

 (PI. XXVII, fig. 2). 



An external sheath occurs in the bundles of the roots of 



1 ' Betrachtungen,' p. 76. 



' Perhaps we may regard the colourless parenchyma investing the bundles 

 iu the leaves of Conifers as constituting a phloem-sheath. 

 ^ Pleromscheide (Sachs), Gemeinsame Seheide (llussow). 



