THE STEM. 33 
to a type of tissue known as sclerenchyma, characterized by thick- 
ened woody cell-walls and absence of protoplasm. In the soft bast 
thereis nothing very 
remarkable about 
the parenchyma, but 
the sieve tubes are 
very important. 
They are the essen- 
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 steve tube 
is made up of a 
series of elongated 
members or joints, 
somewhatswollen at 
their ends, and sepa- 
rated by transverse 
partitions, the sieve 
plates perforated (as 
we have seen in the 
cross - section) by 
numerous pores 
(fig. 8*). We have 
again to deal, in all 
probability, with 
cell-derivatives. 
Each member was originally one of a row of cells. 
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Fig. 6.—Transverse and Radial Sections of Sunflower Stem, to 
show structure of a Vascular Bundle. [After Prantl.] Much 
magnified. B.sh. bundle sheath; P. pith; co. cortex; YX. 
xylem; ss. spiral vessels; tt. pitted vessels; 2.f. xylem 
fibres ; Ph. phloém; s.t. sieve tubes; ph.f. phloém fibres; 
CF, C.i. cambium, 
The walls 
of the mature tubes, although thickened, remain of cellulose, but 
the nuclei seem to have disappeared. Hach member is lined 
by slimy protoplasm, 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, how- 
ever, 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 proto- 
plasm in one member connects with that in the adjoining 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 
