COMPOSITION OF FIBRO-VASCULAR BUNDLES OF PLANTS. 395 



to prove that these so-called bast fibres of Monocotyledons 

 belong in reality to the ground tissue. 



Mohl's view was supported and further developed by 

 Schultz-Schultzenstein,^ who wen4 so far as to suggest that 

 since the bast fibres Avere a highly differentiated form of 

 ground tissue^ they oughts therefore, to be no longer regarded 

 as belonging to the fibro-vascular system. Russow^ regards 

 these prosenchymatous cells as sheath cells belonging to the 

 ground tissue. Schwendener^ has demonstrated the correct- 

 ness of the views of his predecessors, and has founded on 

 them a new theory of the general composition of fibro- 

 vascular bundles. He brings forward examples to show that 

 this bast tissue bears no definite morphological relation to 

 the other elements of the vascular bundles, and he points out 

 that a well-marked layer (bast sheath) often intervenes (in all 

 Cyperaceae and JuncaceEe and in many Graminese, PI. XXVII, 

 fig. 1) between the bast and the vascular bundle, consisting of 

 cells, the walls of which are generally somewhat thickened and 

 resist the action of concentrated sulphuric acid, thereby indi- 

 cating that they can be only very slightly permeable by water. 

 These cells difier from the bast cells in the different form 

 and arrangement of the pores in their walls, and frequently 

 in that they are parenchymatous. To the mass of tissue 

 lying within this layer, which is, in fact, a fibro-vascular 

 bundle without any hard bast, he gives the name of Mestom. 

 He finds, further, that thick-walled cells, similar to the bast 

 cells, occur in the cambiform tissue in various palms, 

 Liliacese and species of Pandanus, and in the ground tissue 

 of some Cyperacese and Graminese. From these facts, and 

 from similar observations upon vascular Cryptogams and 

 Dicotyledons, which we have yet to consider, he is led to the 

 conclusion that these bast cells are arranged in obedience to 

 mechanical and not to morphological laws ; that they may 

 be developed, in fact, in any tissue which requires support. 



It has already been said that in connection with the 

 secondary fibro-vascular bundles of those Monocotyledons 

 which grow in thickness, we find cells occupying that posi- 

 tion in relation to the Mestom which is occupied by the bast 

 cells in other Monocotyledons, from which they differ in 

 structure (fig. 4). Schwendener regards these pitted cells as 

 being the analogues of the bast cells, whereas Russow* con- 



' ' Cyklose des Lebenssaftes,' 1841, p. 245. 

 - Loc. cit., p. 171. 



* *Das Mecb. Prinzip.,' p. 16. 



* ' Betracbtungen,' p. 6. 



VOL. XVI."— NEW SER. C C 



