


56 FLOWERING PLANTS. 
it is from the pericycle fibres of different plants that hemp, 
- flax, and kindred substances are manufactured. Within the 
bast fibres, among the bast parenchyma, will be seen the szeve- 
tubes (s.t). These are formed in much the same way as vessels, — 
with this difference: the cell-walls between the adjoining 
cells do not entirely disappear, but become perforated, so that 
if the section passes through the cavity of the sieve-tube it 
appears round and hollow, just as a vessel would do ; but if it 
passes through the perforated wall, this appears as a plate 
- within the vessel, and in a transverse section of the stem has 
a punctate appearance (s.p). Each sieve-tube has in connec- 
tion with it quite small cells, rich in protoplasm and with a 
well-marked nucleus. ‘They are called companion cells ; they 
are distinguished from the cells forming the bast parenchyma 
by their small size. It will now be clear that in a sunflower 
stem bast consists of— 
1. Bast fibres ; these are prosenchymatous. 
2. Sieve-tubes with companion-cells. 
3. Bast cells, or bast parenchyma. 
It is through the sieve tissue that the nitrogenous food -passes 
from one part of the plant to another. : 
The cambium (cb) consists of meristematic cells, arranged 
regularly one above the other, longer than they are broad, 
with very thin walls. The function of the cambium will be 
explained later in connection with the growth of the stem in 
_thickness. : | 
The wood of a fibro-vascular bundle, like the bast, con- 
sists also of fibres, cells, and vessels, but there is this im- 
portant difference: in the wood these structures have been 
thickened by lignin, so that the cell-walls do not merely 
consist of cellulose, as in the bast, but have become lignified. 
The wood vessels are practically hollow tubes, conducting 
water and substances in solution in it through the plant; 
whereas the sieve-tubes are not complete tubes, owing to the 
sieve-plates. In a transverse section the vessels are cut 
