48 



Elementary Botany 



stem increases in thickness, but also fresh phloem and xylem 

 are produced between the original bundles. 



5. Outside the cambium there is the interrupted ring of 

 phloem, liber, or inner bark. Like the wood this consists of 



three elements, viz. : — 



a. The bast vessels or sieve- 

 tubes (fig. 36). 



b. Bast fibres (fig. 38). 



c. Bast parenchyma or soft 

 bast. 



The separate annual layers 

 of the liber cannot, as a rule, 

 be so readily distinguished as 

 those of the wood, they being 

 much thinner and compressed 

 together by the growth in 

 thickness of the tree. In some 

 cases, however, the liber can 

 be separated into thin plate- 

 like layers. 



A good example of this is 

 to be seen in the Lace-bark tree 

 {Lagetta lintearid) of Jamaica, 

 where the inner bark separates 

 into thin sheets having the 

 appearance of lace, the holes 

 in it being the* perforations for 

 the passage of the medullary 

 rays. 



The fibres of the bast act 

 as sap circulators, bringing 

 down the elaborated sap from 

 the leaves. 



6. The green layer, or 

 phelloderm, consists of chlorophyll-containing cells, often inter- 

 mixed with laticiferous vessels. 



7. The outer or cork layer with its formative phellogen 

 immediately beneath it. These two last layers form a pro- 



Fig. 70. — Part of a transverse section 

 through the bast of the Wild Lettuce 

 {Laciuca scariola) : B/i babt fibres : b/, 



, bast parenchyma ; m" outer, m' inner 

 lati jif erous vessels ; R^, cortical paren- 

 chyma ; Hi/, wood-fibres. 



