MKCIIANISMS FOn COWKV.WCE TO AXn FRO. 



4G9 



across the trunk of a fir or of a leafy ti'ee, it is seen that in most cases the vascular 

 Inintllcs in tlie cross section are so arranged that they form togctlier a ring rouml 

 the central pith. This ring appears intorrupteJ by the tissues just described, which 

 radiate out from the medulla; and thus is explained their name, medullary rays. 



Soft bust is to be considered as a third form of conducting mechanism for the 

 organic compounds formed in the green cells. It consists partly of thin-walled 

 parenchymatous cells, and frequently also of long, narrow cells tapering at the ends 

 (cambiform cells), which are elongated in the direction of the bundle or strand to 



3 4 



Fig. 125.- 



5 6 7 8 9 10 11 12 13 



-Portion cut from a Branch of a Leafy Tree ; x about 200. Diagrammatic. 



■ Superficial coat (Epidermis). = Cork (Periderm). ' Cortical parenchyma. * Vascular bundle sheath. ' Hard bast, s |:,,s[, 

 parenchyma. ' Sieve-tubes. » Cambium. ' Pitted vessel "> Wood-parenchyma. " Scalariform vessels, is Medullary 

 sheath. IS Medulla or pith. 



which they belong, and form a tissue called the bast parenchyma (see fig. 125*). 

 The other part of it consists of tubes which contain w-alls separated by com- 

 paratively large intervals, often measuring 2 mm., usually placed horizontallj^ but 

 often obliquely, which give the tubes a jointed appearance. Sharply -defined 

 perforated areas appear on the interpolated horizontal walls as w'ell as on the 

 lateral walls of the tubes, they have a sieve-like aspect, and are called sieve plates, 

 the tubes themselves being called sieve-tubes, bast-tubes, or bast-vessels (fig. 125'). 

 The soft bast but rarely forms isolated strands, as, for example, in some Melasto- 

 maceiie; as a rule, strands of firm, elastic, string-like, hard bast cells adjoin it, but 

 these have nothing to do with the transportation of materials, and have merely 

 a mechanical significance (see fig. 125*). 



This fibrous or hard bast, together with the soft bast, forms in very many plants 



