80 



BOTANY. 



regarded as undeveloped sieve ducts, and hence the tissue 

 they form may be included under sieve tissue. Latticed 

 cells are thin -walled and elongated ; they differ from true sieve 

 ducts principally in being of less diameter, and in having 

 the markings but not the perforations 

 of sieve discs. Both of these differences 

 are such as might be looked for in un- 

 developed sieve tissue. 



1O6. In the corres- 

 ponding parts of the vas- 

 cular bundles of Gymno- 

 sperms and Pterido- 

 phytes a sieve tissue is 

 found which differs 

 somewhat from that in 

 Angiosperms. In Gym- 

 nosperms the sieve discs, 

 which are of irregular 

 outline, occur abundant- 

 ly upon the oblique ends 

 and radial faces of the 



Fig. 69. Longitudinal broad tubes (Fig. 70). 



In Pteridophytes the 

 tubes have varying 



closed lip by the thickening f nvrn Q . , 'Pnti'i<ipttmt 

 of its substance. X 400. AqUlSemm 



After DeBary. an Ophioglossum they 



are prismatic, with numerous horizontal but 

 not vertical sieve discs ; in Pteris and many 

 other ferns they have pointed extremities, 

 and are greatly elongated, bearing the sieve 

 discs upon their sides (Fig. 71). In the 

 larger Lycopodiacc-CB the sieve tubes are pris- 

 matic and of great length ; in the smaller 



viewof the~end a of a a 



species there are tissue elements destitute of Pjf, e * 



sieve discs, but which are otherwise, includ- posed of many little 



. ' iTii punctured areas 



ing position in the stem, exacply like the grouped togetnerir- 



-. piii regularly, x 375. 



sieve ducts of the larger species. After DC Bary. 



(a) Good specimens of sieve tissue may be obtained for study by 

 making longitudinal sections of the stems of Cucurbita, Cucumis, 



