MECHANISMS FOR CONVETANCE TO AND FRO. 



409 



in 



the 



pith, 

 rad^ateout fro , the ^'^^Z^Z^ZS^ 



Soft bast u, to be considered as a third form of conducting mechanism fo 'the 

 orgamc compounds formed in the green cell, It consists partly of thin-wal ed 

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

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



345678 9 10 11 12 IS 



Fig. 125. Portion cut from a Branch of a Dicotyledon; x about 200. Diagrammatic. 



i Superficial coat (Epidermis). 2 Cork (Periderm). * Cortical parenchyma. < Vascular bundle sheath. Hard bast Bast 

 parenchyma. T Sieve-tubes. Cambium. Pitted vessel. "Wood-parenchyma. u Scalarifonn vessels. "Medullary 

 sheath, is Medulla or pith. 



which they belong, and form a tissue called the bast parenchyma (see fig. 125 8 ). 

 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 7 ). 

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

 maceae; 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 5 ). 



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



