COMPOSITION OF FIBRO-VASCULAR BUNDLES OF PLANTS. 391 



many Dicotyledons (Pomese, Papilionese, &c.), and in those of 

 all Conifers, except Ephedra and the Abietinese.^ 



It is interesting to find that, in Dicotyledons, in which 

 growth in thickness continues, the cells of the bundle sheath 

 very rarely become thickened (except roots of Ranunculus 

 acer,cJiius, lingua, ?t.\\f\ others), forming the " Steifungscheide" 

 of Russow, whereas in Monocotyledons this frequently takes 

 place in the rhizomes and roots. 



We have now briefly traced the range and distribution of 

 these sheathing cells, and it remains for us to consider what 

 is their morphological value. As regards the external sheath, 

 there can be no doubt that its elements are extremely modi- 

 fied forms of the cells of the ground tissue. Russow^ refers 

 the bundle sheath and the phloem sheath, in fact, his 

 " Koleochyma," as well as the external sheath, to the 

 ground tissue, and herein he is followed by Sachs.^ Falken- 

 berg,* however, cannot accept this view in its entirety. He 

 mentions that, in Hedychium and Canna, when some of the 

 bundles bend outwards into the leaves they lose their true 

 vascular elements, and consist only of bast fibres; and so, on 

 E-ussow's view, we would have bundles which terminated 

 blindly at their upper extremities, their sheaths alone form- 

 ing their continuation. He goes on to say that such an in- 

 dependent origin of sheaths is an impossibility, unless the 

 distinction between the vascular and ground tissues be given 

 up. He concludes that so long as this distinction remains 

 we must consider the sheaths of Monocotyledons at least as 

 belonging in some cases to the ground tissue, in others to 

 the vascular tissue. On the whole, however, the balance of 

 evidence seems to incline towards Russow's view, and it is 

 to that view that reference will be made throughout this 

 paper. We may now go on to consider the composition of 

 the fibro-vascular bundle itself 



In the year 1858 Nageli published a work on this subject,^ 

 in which, for the first time, a definite and satisfactory account 

 was given of the structures in question. He regarded the 

 fibro-vascular bundles of Dicotyledonous plants as consisting 

 of two principal parts, an outer and an inner, separated by 

 the layers of delicate cells forming the cambium, the outer 



' Strassburger, 'Die Coniferen u. Gnetaceen,' 1872, p. 346. He terms 

 this structure " aeussere Schutzscheide." 



2 ' Vergl. Untersuch.,' p. 166. 



3 Loc. eit., p. 126. 



" ' Vergl. Unters. ueb. d. Bau d. Vegetationsorgane,' 1876, p. 142. 

 8 ' Beitrage zur Wissenscbaftlichen Botanik,' Heft i, von Carl Nageli, 

 Prof, in Miinchen. 



