SECT. I 



MORPHOLOGY 



149 



certcain cases, of bast fibres. These elements of the bast generally 

 form alternating bands. 



The Araucarieae, Taxineae, and Cupressineae have definite, vertical rows of 

 bast parenchyma cells which take the place of the companion cells wanting in 

 these plants. At a certain distance from the cambium the sieve-plates become 

 overlaid by callus. During the vegetative period following their development, 

 the sieve-tubes become empty and compressed together (Fig. 151 B, cv). Less 

 often as in the Vine the sieve -tubes remain functional for more than one 

 year ; the callus is re- 

 moved when their activ- 

 ity is resumed. The rows 

 of bast parenchyma cells 

 containing albuminous 

 substances which are 

 found in some Conifers, 

 undergo disorganisation 

 at the same time as the 

 adjacent sieve-tubes; the 

 bast parenchyma cells 

 which contain starch, 

 on the other hand con- 

 tinue living for years, 

 and even increase in size, 

 while the sieve - tubes 

 become compressed. 



The elements of 

 the phloem tissue in- 

 cluded in the bast 

 strands of woody 

 Dicotyledons (Fig. 

 159) are represented 



by SIEVE-TUBES {v) 



and COMPANION 



CELLS (c). To the par- 

 enchymatous tissiies 

 of the bast belong BAST parenchyma {p), BAST fibres (/) and transi- 

 tional 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 frequently exhibit a great regularity 

 in their arrangement. Thus in the Lime (Fig. 159) there is a regular alternation 

 of sieve-tubes (?') and companion cells [c), bast parenchyma containing starch (p) 

 and crystals (^•), tangentially flattened bast parenchyma {p), and then another zone 

 of sieve-tubes. The sieve-tubes of Dicotyledons as of Gymnosperms only remain 

 functional for a short time, after which they liecome empty and crushed. The 

 companion cells, which are sister cells of the sieve-tubes, undergo the same fate. 

 The starch-containing parenchy na, on the other hand, may remain for years 



Fig. 157. — Portion of a transverse section of the woort of Tilia 

 ulmifolia. vi, Large pitted vessel ; t, tracheides ; I, wood 

 fibre; p, wood parenchyma ; )•, medullary ray. (x 540.) 



