80 



BOTANY. 



regarded as undeveloped sieve ducts, and lience 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. 



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, occur abundant- 

 ly upon the oblique ends 

 and radial faces of the 

 Longitudinal broad tubes (Fig. 70). 

 ^^^g^tirof The^gVp^e! In Pteridophytes the 

 SrfSf?"^ve"'&^^ tubes have varying 

 ^i?i?a^s,?bsfaL''cl'''*«!}rifo™s; in Equisetum 



After DeBary. g^XlA OpMogloSSUm they 



are prismatic, with numerous horizontal but 

 not vertical sieve discs ; \ti Pteris and many 

 other ferns they have pointed extremities, 

 and are greatly elongated, bearing the sieve 

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

 larger Lyaipodiacem the sieve tubes are pris- 

 matic and of great length ; in the smaller Srsfem^YhVive 

 species there are tissue elements destitute of P'*Jf ^ ^''^'''i^^i?,'' 



^ erally ana are com- 



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



,, , ,, ,-, ,, punctured areas 



mar position m the stem, exactly like the grouped togethenr- 



.^ ^ , „ ,, 1 . ■' regularly, x 375- 



sieve ducts of the larger species. After De Bary. 



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

 making; longitudiual sections of tlxe stems of Oucurbita, Cucvmii, 



Fig. 70. — Radial 

 view of the end of a 

 sieve tube of Segu'da 

 taken 



