DIV. I 



MORPHOLOGY 



159 



have strongly -thickened walls that may be lignified or not. The 

 parenchymatous cells are elongated in the direction of the strand ; 

 they are living cells with abundant reserve materials and thin 

 unlignified walls. 



At a certain distance from the cambium the sieve-plates become overlaid by 

 callus. During the vegetative period following their development the sieve- 

 tubes become empty and compressed together (Fig. 175 B, cv). Less often, as in 

 the Vine, the sieve-tubes remain functional for more than one year ; the callus is 

 removed when their activity is resumed. The rows of bast parenchyma cells 

 containing albuminous substances which are found in some Conifers undergo 

 disorganisation at the same time as the adjacent sieve-tubes ; the bast parenchyma 



FIG. 184. Portion of a transverse section of the bast of Tilia ulmifolia. v, Sieve-tubes ; v*, 

 sieve-plate ; c, companion cells ; t, cells of bast parenchyma containing crystals ; p, bast 

 parenchyma ; I, bast fibres ; r, medullary ray. (x 540. After STRASBURGER.) 



cells which contain starch, on the other hand, continue living for years, and even 

 increase in size, while the sieve-tubes become compressed. 



B. Arrangement of the Tissues in the Bast. This re- 

 sembles the arrangement in the wood. The strands of sieve-tubes 

 form branched tracts in which the sieve- tubes have a continuous 

 course from the roots to the foliage. The sieve-tubes, and the 

 longitudinally-running bast parenchyma, are related at intervals to the 

 medullary rays (Fig. 179 ms"\ which have been seen to be the 

 continuation of the medullary rays of the wood. Thus the products 

 of assimilation from the foliage can either pass in the bast towards 



