VEGETATIVE OKGANS OF PLANTS. 303 



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 represent the disk-pore seen flatwise, as in 

 Fig. 53. At e the section passes through the new 

 annual ring into the autumn wood of the preceding year. 

 Sieve-cells, or Sieve-ducts. The spiral, ring, and 

 dotted ducts and pitted wood-cells already noticed, ap- 

 pear only in the older parts of the vascular bundles, and, 

 although they may be occupied with sap at times when the 

 stem is surcharged 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 independ- 

 ent set of ducts, the so-called sieve-tubes, which are usu- 

 ally near to and originate from the cambium. These 

 are extremely delicate, elongated cells, whose transverse 

 or lateral walls are perforated, sieve-fashion (by absorp- 

 tion 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 occupied with air alone. These 

 sieve-ducts are believed to be the channels through which 

 the organic matters that are formed in the foliage mostly 

 pass in their downward movement to nourish the stem 

 and root. Fig. 55 represents the sieve-cells in the over- 

 ground stem of the potato ; A, B, cross-section of parts 

 of vascular bundle ; A, exterior part towards rind ; B, 

 interior portion next to pith ; a, a, cell-tissue inclosing 

 the smaller sieve-cells, A, B, which contain sap turbid 

 with minute granules ; b, cambium cells ; c, wood-cells 

 (which are absent in the potato tuber) ; d, ducts inter- 

 mingled with wood-cells. C represents a section length- 

 wise of the sieve-ducts ; and D, more highly magnified, 

 exhibits the finely perforated, transverse partitions, 

 through which the liquid contents more or less freely 



