THE STEM. 33 
o a type of tissue known as sclerenchyma, characterized by thick- 
ened hard cell-walls and absence of protoplasm. In the soft bast 
care cama eee 2 a Che g a, 
remarkable about Gi : cs 
the parenchyma, but oe FEISS RA SKS Ca. 
ee SAIN ES é 
the sieve tubes are COE ER SS eee de) J 
very important. maser <6,  ’ Disees ere A 
They are the essen- Nl 
tial part of the bast, 
and are always pre- 
sent in it, at any 
rate in phanero- 
gams. The radial 
section will show, if 
carefully prepared, 
that each sieve tule 
is made up of a 
series of elongated 
members or joints, 
somewhatswollenat 
their ends, and sepa- 
rated by transverse 
partitions, the sieve 
plates perforated (as 
we have seen in the 
cross - section) by 
numerous pores. 
a : Fig. 6.—Transverse and Radial Sections of Sunflower Stem, to 
We hay e again to show structure of a Vascular Bundle. [After Prantl.] Much 
deal, in all proba- ae a. ices sheath Hdd gee x cortex eae 
“7: : : xylem; ss. spiral vessels; tt. pitted vessels; 2.7. xylem 
bility ’ with cell-deri- fibres ; Ph. phloém; s.t. sieve tubes ; ph.f. phloém fibres ; 
vates. Each mem- C.J, C.i. cambium. 
ber was originally one of a row of cells. The walls of the 
mature tubes, although thickened, remain of cellulose, but the 
protoplasm seems to have disappeared. Each member is lined 
by a slimy substance, which is denser, and therefore stains 
more deeply than the rest. The transverse party-walls between 
the original row of cells appear to have been thickened, pits, 
however, having been left, which corresponded on opposite 
sides. Hence at any spot where these occurred there would be 
a pit or depression on either side, the two being separated by 
the pit-membrane, absorption of which would lead to formation 
of a little canal, piercing the sieve plate. In the sieve tube the 
slime in one member is connected with that in the adjoming ones 
by threads of the same material running through the sieve plates. 
We have here an example of a cell-fusion or vessel, which terms 
Cc 
NA, 
l 
WN 
WA? 
WAYS 
