392 SYDNEY H. VINES. 



being the bast or phloem, the inner the wood or xylem. 

 The phloem he found to be made up of parenchyma, of liber 

 cells or bast fibres, of soft bast, and of latticed cells ; the 

 wood, of vessels, of woody fibres, and of parenchyma. He 

 explained the structure of the fibro-vascular bundles of Mono- 

 cotyledonous plants in essentially the same way. He found 

 that here also they consisted of an inner xylem composed of 

 vessels and woody cells, and of an outer phloem composed of 

 bast fibres. He found that they were separated, not, indeed, 

 by layers of cells capable of undergoing division constituting 

 a cambium, but by an aggregation of delicate cells, to which 

 he gave the name of cambiform. Moreover, in the bundles 

 of these plants, this separation of fhe xylem from the phloem 

 by the intervention of the cambiform tissue is confined only 

 to the central part of the bundle ; towards its circumference, 

 on each side, the elements of the xylem and of the phloem 

 imperceptibly blend (fig. 4). 



These views of the general structure of fibro-vascular 

 bundles met with very general acceptance, and, with some 

 modifications, are still received as an adequate expression of 

 our knowledge of the subject. According to a recent ex- 

 position of tliem,! we find that the terms xylem and phloem 

 are retained, and are applied in the same sense in which 

 Nao-eli first employed them, but that, as the result of sub- 

 sequent investigations, the account now given of the his- 

 tological elements of which the phloem and the xylem 

 respectively consist, difiers somewhat from that given by 

 Nageli. 



Sachs states that both the xylem and the phloem consist 

 of three forms of tissue: (1) combinations of cells to form 

 vessels (vessels of the xylem, sieve tubes of the phloem) ; 

 (2) prosenchymatous cells (woody fibres of the xylem, bast 

 fibres or hard bast of the phloem) ; and (3) parenchymatous 

 cells (wood parenchyma of the xylem, soft bast or bast 

 parenchyma of the phloem). 



In comparing these two accounts we find that the 

 vascular elements of the xylem are similarly described in 

 each, whereas those of the phloem are termed, in the one 

 latticed cells, in the other sieve tubes. This diflference of 

 name is due to the fact that it is not always easy to detect 

 the perforations in the septa between the cells which are 

 characteristic of sieve tubes, and serve to distinguish them 

 from the latticed cells, the cavities of which are not in com- 

 munication. With regard to the prosenchymatous elements, 

 we find a complete agreement, and this is the case also with 

 ' baclis, loc. cit.j p. 112. 



