280 HOW CROPS GROW. 
these pore disks is made evident. The section, likewise, 
gives an instructive illustration of the general character 
of the simplest kind of wood. A, are the young cells of 
the rind; @, is the cambium, where cell multiplication 
goes on; W, is the wood, whose cells are more developed 
the older they are, i.e., the more distant from the cam- 
bium, as is seen from their figure and the thickness of 
their walls. At @ is shown the disk in its earliest stage; 6 
and ¢ exhibit it in a more advanced growth before it be- 
comes a pore, the original cell-wall being still in place. 
At d, in the finished wood-cells, the disk has become a 
pore, the primary membrane has been absorbed, and a free 
channel made between the two cells. The dotted lines at 
@ lead out laterally to two concentric circles, which repre- 
sent the disk-pore seen flatwise, as in fig. 53. At ¢, the 
section passes through the new annual ring into the au- 
tumn wood of the preceding year. 
Sieve-cells or sieve-ducts,—The spiral, ring, and dotted 
ducts and porous wood-cells already noticed, appear only 
in the older parts of the vascular bundles, and although 
they are occupied with sap at times when the stem is sur- 
charged with water, they are ordinarily filled with air 
alone. The real transmission of the nutritive juices of the 
growing plant, so far as it goes on through actual tubes, is 
now admitted to proceed in an independent set of ducts, 
the so-called sieve-cells, which are usually near to, and 
originate from the cambium. These are extremely deli- 
cate, elongated cells, whose transverse or lateral walls are 
perforated, sieve-fashion, (by absorption of the original 
membrane,) so as to establish dirett communication from 
one to another, and this occurs while they are yet charged 
with juices and at a time when the other ducts are occu- 
pied with air alone. These sieve-ducts are believed to be 
the channels through which the matters organized in the 
foliage most abundantly pass in their downward move- 
ment to nourish the stem and root. Fig. 55 represents 
