80 



BOTANY. 



regarded as undeveloped sieve ducts, and hence the tissue 

 they form may be included under sieve tissue. Latticed 

 flells are thin-walled and elongated ; they difEer 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 difEerences 

 are such as might be looked for in un- 

 developed sieve tissue. 



106. — 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, occiir abundant- 

 ly upon the oblique ends 

 and radial faces of the 

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



youffgba'rk^of'the "grape! I^ PteridophyteS the 

 {YitiB vlnifera), teken_ in ^^^^^ ^^^^ 



winter ; the sieve plate is 



Yarymg 



?ri?B"&sfa;^cl*"x''mi forms; in Equisetum 



After De Bary. 



and Ophioglossum they 

 are prismatic, with numerous horizontal but - 

 not vertical sieve discs ; in Fter is and many 

 other ferns they have pointed extremities, 

 and are greatly elongated, bearing the sieve 

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

 larger Lycopodiacece the sieve tubes are pris- 

 matic and of great length; in the smallei old stem ; the sieve 

 species there are tissue elements destitute of plates are placed lat- 



t ,. 111.1 ,1 • -,, erally and are com- 



Bieve discs, but which are otherwise, mclud- posed of many mtie 



... ■ Ji J. J.1 Ti ,1 punctured areas 



mg position m the stem, exactly like the grouped togetherir- 



sieve ducts of the larger species. After^DeBary.^™" 



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



making longitudinal sections of the stems of CucurUta, Cucumis, 



Kg. 70. — Eadial 

 view of the end of a 

 sieve tube of Sequoia 

 gigantea^ taken 

 in the smaller *rom the bark ot an 



