394 SYDNEY H. VINES. 



and xylem as forming a common continuous tissue of similar 

 cells (Geleitzellen or Leitzellen), which invest the vessels of 

 the bundle. 



Now that we have gained some definite idea of the meaning 

 of the words " sheath" and " fibro-vascular bundle," we may 

 go on to inquire into the relations existing between them. 



Taking the fern (Pferis aquilina, fig.l, p. 389) as an example 

 of the vascular Cryptogams, we find that here the relations 

 between sheath and bundle are of the simplest. In the 

 central portion of the bundle we have the xylem, around it 

 we have the layers of phloem, and investing these we have 

 the tAvo layers of sheathing cells. 



Passing on to a similar consideration of the bundle in 

 those Monocotyledons, the stems of which do not increase in 

 diameter, we find a xyliem forming the inner part of it, a soft 

 bast lying externally to this, and then a mass of thick- walled 

 prosenchymatous cells quite at the exterior, which invests 

 more or less of the circumference of the bundle (PI. XXVII, 

 fig. 3) . Nageli regards these cells as forming the true bast por- 

 tion of the bundle. DippeP considers that those cells which 

 lie on the peripheral side of the bundle are bast fibres, whereas 

 those which lie on its central side are woody fibres, and this 

 view coincides with those of linger^ and of Schacht.^ But on 

 referring to the account given of the bundle sheath we find 

 that these cells have been claimed as belonging to the ground 

 tissue. Von Mohl,* as early as 1833, doubted the correct- 

 ness of the view that all prosenchymatous cells belonged 

 to fibro-vascular bundles. He says : " We are accustomed, 

 in Phanerogams, to meet with thick-walled prosenchymatous 

 cells only in the fibro-vascular bundles, and we allow our- 

 selves, therefore, to be easily led into regarding every aggre- 

 gation of elongated cells as either wood or bast," and in his 

 account of the structure of the palm stem,^ speaking of this 

 thick-walled prosenchyma, he goes on to say, in opposition 

 to Link" and to Kieser,'^ that these cells, in their thick walls, 

 their small width, and their elongated form, resemble bast 

 cells, but that it would be a grave error to compare this mass 

 of tissue with the bast of Dicotyledons. This latter erro- 

 neous view he supports by several arguments, and he goes on 



1 ' Das Mikroskop./ ii, p. 210, 1869. 



^ 'Anat. u. Phjs. d. Pflanzen,' p. 218, 1855. 



^ ' Grundriss der Anat. u. Pbys.,' p. 64, 1859. 



•» ' Vermiscbte Schriften,' p. 116, 3845. 



^ Loc. cit., p. 150. 



« • Grundlehren,' p. 143, 1807. 



' 'Grundziige der Anat.,' p. 72, 1815. 



