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. JR, are the young cells of 

 the rind ; C, 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 a is shown the disk in its earliest stage ; 

 and c 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 

 d lead out laterally to two concentric circles, which repre 

 sent the disk-pore seen flatwise, as in fig. 53. At 0, 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 wheii 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 direct 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 representg 



