SECT. I 



MORPHOLOGY 



139 



of woody Dicotyledons (Fig. 150) are represented by SIEVE-TUBES (v) 

 and COMPANION CELLS (c). To the parenchymatous tissues of the 

 bast belong BAST PARENCHYMA (p), BAST FIBRES (/) and transitional 

 forms between them. 



The bast fibres,' like the fibres of the wood, may occur in an unthickened' form 

 as FIBROUS CELLS, either with or without living contents, or they may be filled 

 with starch, or finally may become septate. 



The elements of the 

 bast of Dicotyledons fre- 

 quently exhibit a great 

 regularity in their ar- 

 rangement. Thus in the 

 Linn- (Fig. 150) there 

 is a regular alternation 

 of sieve -tubes (v) and 

 companion cells (c), bast 

 parenchyma containing 

 starch (p) and crystals 

 (Jc), tangentially flat- 

 tened bast parenchyma 

 (p), and then another 

 zone of sieve-tubes. The 

 sievo-tubes of Dicotyle- 

 dons as of Gymnosperms 

 only remain functional 

 for a short time, after 

 which they become 

 empty and crushed. The 

 companion cells, which 

 are sister cells of the 



sieve-tubes, undergo the yiff ^ f 



same fate. The starch- 

 containing parenchyma, 

 on the other hand, may 

 remain for years un- 

 altered. The differences 



in the appearance of the bast of dicotyledonous trees are due to the greater or less 

 diameter of' the sieve-tubes, the presence or absence, of bast' fibres, and the arrange- 

 ment of the various elements. 



The Medullary Rays of the Gymnosperms (Fig. 139 ms) and 

 woody Dicotyledons (Fig. 146 pm, sm) form radial bands, composed 

 wholly or in part of parenchymatous elements. Their function is to 

 supply the cambium and wood with the products formed in the leaves 

 and conveyed away by the bast ; they also conduct water outwards 

 from the xylem. The medullary rays in this way link together by 

 radial bands of living cells the protoplasm-containing elements of the 

 bast and wood, thus uniting all the separate living tissues of the stem. 

 The medullary rays are in turn accompanied or, if many-layered, 



!'[<;. 148. Portion of ;i transverse section of the wood of 'Cilia 

 idmifolia. TO, Large pitted vessel ; t, tracheides ; I, wood 

 fibre ; p, v.ood parenchyma ; r, medullary ray. ( x 540.) 



