THE VASCULAR TISSUES. 63 



(6) The bast-fibres are situated outside the phloem proper, 

 and are individually elongated sclerenchymatous elements, which 

 form a layer of varying thickness ; in the stem of Pinus they 

 are oval and compressed when examined in transverse section, 

 and possess minute "pits" in their thick walls. Bast-fibres are 

 not formed as such, annually by the caml>ium, but result from 

 the modification of elements formed in previous years. Function- 

 ally they serve as a protective and supporting layer to the more 

 delicate phloem lying internal to them (see Fig. 53, pl^). 



B. The Xylem. 



The elements formed by the cambium in Dicotyledons and 

 Conifers upon its inner aspect — viz., the rudimentary xylem — 

 are at first elongated thin-walled cells (prosenchyma), which, 

 however, soon undergo the following modifications : — 



1. A general thickening' and chemical change in the cell-wall, 

 known as llgnlflcation (deposit of lignin). 



2. The production of localised areas of thickening, the inter- 

 mediate portions remaining thin (thin wall-areas). 



3. The thin wall-areas later on often became absorbed in adjacent 

 portions of cell-wall, leading to the formation of actual pepforatlons 

 or "pits." At times, however, a thin partition remains unabsorbed, 

 this being usually formed by the middle lamella, which is, in reality, 

 an intercellular substance. 



During these changes the cell undergoes an elongation, but, as 

 a rule, not much increase in its other dimensions. At times the 

 adjacent end-walls of elements may become absorbed, leading to 

 the formation of vessels of relatively great length; or the 

 elements may remain single, the end-walls persisting, when they 

 are known as tracheides. 



The best method of classifying wood-elements is by means 

 of the various thickenings and " pits " occurring on their walls, 

 and in this manner it is possible to distinguish the following 

 varieties : — 



(a) Tracheides or vessels with simple " pits " in their walls. — 

 The pits are at first only thin wall-areas, but subsequently the 

 middle lamella may become absorbed, leading to definite 

 perforations (see Fig. 42, a). 



