THE PHLOEM. 



'39 



Finally, as to the elastic fibres of the vascular bundle already mentioned. 

 These occur in the phloem as true bast-fibres, in the xylem as wood-fibres : 

 the latter have been distinguished from the fibre-Hke trachei'des more particularly 

 as the libriform fibres of the wood. At another opportunity we shall see how, 

 in cases where subsequent growth in thickness of the shoot-axes occurs (i. e. in 

 the proper development of wood) the libriform fibres contribute to the for- 

 mation of the true wood; and, so far as the true bast-fibres are concerned^ it 



Fig. 145.— Transverse section of the phloem 

 of a fibro-vascular bundle in the stem of Cticiir- 

 />ila Pepo (X 550). j;' septa of young sieve-tubes 

 showing areolae — the pores not yet developed j 

 /, / phloem parencliyma ; c,c cambium. There 

 are no bast fibres here, the whole phloem con- 

 sisting of soft bast. 



Fig. 146.— Longitudinal section of the phloem of C/<- 

 curbita Pepo; showing three sieve-tubes, the transverse 

 septa (^, q) of which are not yet perforated. The slimy 

 mass \sl and ps) contained in them is contracted, si a 

 young sieve-plate on the lateral wall : pores will also be 

 found later at x and /. z narrow parenchymatous cells 

 between the sieve-tubes. 



may here be pointed out simply that they are wanting in many vascular bundles, 

 and in other cases appear more isolated ; in others, again, they exist in the form 

 of layers or thick strands on the outside of the phloem. 



The peculiar characteristics of the vascular bundle lie, according to my view, 

 which agrees with that of De Bary, in the presence of tracheal elements in the 

 xylem, and of sieve-tubes, or of tissue-elements similar to them, in the phloem. 



» It will be clear from the text and from note 2, that by 'bast-fibres' are not to be understood 

 here the Stereom elements distinguished as ' bast ' by Sch wendener. 



