MECHANISMS FOR CONVEYANCE TO AND FRO. 



469 



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

 bundles in the cross section are so arranged that they form together a ring round 

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

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



Soft bast 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 



1234 5 6 r S lU 11 12 13 



Fig. 125.— Portion cut from a Branch of a Leafy Tree ; x about 200. Diagrammatic. 



I Superficial coat (Epidermis). 2 Corli (Periderm). ' Cortical parenchyma. * Vascular bundle sheath. * Hani bast. 6 Bast 

 parenchyma. 7 Sieve-tubes. » Cambium. 9 Pitted vessel. 10 Wood-parenchyma. 11 Scalariform vessels. 12 Medullary 

 sheath. i3 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 walls separated by com- 

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

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

 perforated areas appear on the interpolated horizontal walls as well 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- 

 macese; 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 



