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 bast 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. 17and18,b), which arehomo- 
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 
