50 



BOTANY OF THE LIVING PLANT 



additions to the tissues already present. Those which lie internally 

 are added to the wood, those externally are added to the bast 

 outside the ring. If this process be continued, the structure of 

 the stem will become like that shown in Fig. 36, C. 



There are three different purposes to be served by the growing 

 vascular column, viz. increased power of conduction, of storage, and 

 of mechanical resistance. These are met by three different types 

 of tissue, and though differing in detail, all of these are to be found 



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Fig. 37. 



Cambium cell of the Lime, and its \'arious products in the secondary wood and 

 secondary bast, all drawn to the same scale, and seen in tangential section. i.= 

 nucleated cambium cell. ii.=fibrous tracheid. iii. = group of cells of wood- ^ 



parenchyma, iv. v. — single lengths of vessels, the obhque terminal walls having 

 been absorbed, vi. vii. = wood fibres, bent to save space in the figure. vlii. = group 

 of bast-parenchyma, ix. — single length of sieve-tube, with oblique terminal walls 

 perforated. x. = bast fibre bent to save space in the figure. ( x 75.} 



as a rule both in the internal woody column and in the external 

 band of bast. The tissues in question are, the vessel for conduction, 

 the parenchymaious cell for storage and other functions, and the fibrous 

 cell for mechanical resistance. The forms of these various products 

 of the cambial cells, as seen in the stem of the Lime, are shown all to 

 the same scale in Fig. 37, i.-x. 



When a large vessel of the wood is to be formed, a longitudinal row 

 of cambial cells widen greatly, pushing aside the adjoining cells. 

 The lateral walls become thickened, and usually pitted, but the end 

 walls are absorbed, and the protoplasm disappears. Thus they become 

 tubes for transit of the sap that 611s them. As they have lost 



