THE ANGIOSPERMAE : STEMS 



89s 



conducting tissues. These fibres closely resemble those which form the 

 fibres of the so-called pericycle, external to the phloem, and are often classed 

 with them as libriform fibres, from the old name " liber " (or " bast "), 

 applied to the inner bark, in which the phloem was distinguished as " soft 

 bast " and the fibres as " hard bast." Fibres of the secondary phloem, being 

 produced from the cambium, show the same radial arrangement as other 

 phloem cells, but the pericycle fibres, coming direct from procambial cells, 

 do not. 



Libriform fibres are narrow and very highly thickened, and their pits 



Fig. 879. — Tilia vulgaris. Transverse section of a woody stem showing wedge- 

 shaped medullary rays in the phloem zone and phloem composed of 

 alternating tangential bands of sieve tubes and thick-walled phloem fibres. 



are verv* small, so that the pit opening shows little or no border. The pit 

 openings are slit-like and the openings on the inside and the outside of the 

 wall are usually at right angles to each other, so that the pits appear like 

 minute crosses on the cell wall. They are longer than the xylem fibres, but 

 flexible and tough rather than brittle. Jute, and several other important 

 fibres consist of this type of cell. 



9. Secondary Vascular Tissue : Xylem. The term metaxylem includes 

 all the secondary xylem. The cell structure of secondary xylem is very 

 variable, and each type of wood has its recognizable characteristics, by 

 which it may be identified. The usual constituents are vessels or tracheae, 

 tracheids, xylem parenchyma and fibres of various types. 



In true vessels the successive elements open into one another in longi- 

 tudinal succession by membraneless portals so that the lumen forms a 



