THE TREE ITS SHOOT-SYSTEM 113 



through which the protoplasmic and other contents of 

 the contiguous segments pass uninterruptedly. Similar 

 sieve-plates occur on the lateral walls of the segments 

 also. The walls are not thickened and not lignified, and 

 thus the morphological similarities between the sieve- 

 tubes of the ba.st and the vessels of the wood (which only 

 contain air and water, have their septa absorbed, and 

 their walls lignified and covered with bordered and simple 

 pits) depends almost entirely on the similar develop- 

 ment. The sieve-pores are very fine, and easily over- 

 looked. 



(3) The bast fibres (figs. 1 7 and 1 8, &), which are homo- 

 logous with the libriform fibres of the wood, and are deve- 

 loped in the same way from single cells of the cambium. 

 They are short, blunt, very thick-walled fibres, grouped 

 in strands which appear on the transverse section of the 

 bast as tangential bands 2-4 deep, alternating (in the 

 radial direction) with broader bands of sieve-tubes and 

 parenchyma. These bands of fibres (hard bast) are 

 accompanied at their outer and inner boundaries by 

 parenchyma-like cells arranged in vertical rows, each 

 of which contains a large simple crystal of calcium 

 oxalate embedded in yellowish substance, and the walls 

 of which are slightly sclerotic. Similar vertical series 

 of cells are found in the soft bast, but they contain 

 compound (clustered) crystals of the same salt (figs. 17 

 and 18, e). 



The soft bast also contains scattered roundish 

 groups of short sclerenchyma cells, the thickened walls 



I 



