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A TEXT-BOOK OF BOTANY. 



The leptome or sieve is distinguished from the other con- 

 ducting elements in that the walls are thin and are composed of 

 cellulose (Fig. 146). It consists of superimposed elongated cells, 

 the transverse walls of which possess numerous pores which are 

 supposed to be in the nature of openings, permitting of the 



in 



FIG. 146. Different forms of sieve pores: I, portion of sieve tube of Bryonia alba, 

 II of Cucurbita Pepo, A longitudinal section and B in transverse section; III, portion of a 

 sieve cell of Larix europaa showing round sieve pores; IV, an old sieve plate in Bryonia 

 alba treated with chlor-zinc-iodide, showing the striated callous plates (c), (z) cell-wall, 

 (s) sieve plate, (i) contents of sieve tube, (h) cell membrane, (c) callous plates. After Dippel 

 in "Das Mikroskop." 



direct passage of the contents from one cell to the other. This 

 transverse wall, which may be either horizontal or oblique, is 

 known as the SIEVE PLATE, and the thin places as pores of the 

 sieve. The sieve plates are sometimes also formed on the longi- 



