22 



BOTANY. 



scribed above. la Conifers the sieve-discs, wliicli are of irregular 

 outline, occur abundantly upon the oblique ends and radial faces_ of 

 the Droad tubes (Pig. 14). .In the Horsetails (Equisetum) and Adder- 

 tongues (Opliioglossum) they are prismatic, with 

 numerous horizontal but not vertical sieve-discs; 

 in Brakes (Pteris) and many other ferns they have 

 pointed extremities, and are greatly elongated, 

 bearing the sieve-discs upon their sides. In the 

 larger Club-mosses the sieve-tubes are prismatic 

 and of great length; in the smaller species there 

 are tissue elements destitute of sieve-discs, but 

 which are otherwise. Including position in the 

 stem, exactly like the sieve ducts of the larger 

 species. 



48. Traeheary Tissue. — Under this head 

 are to be grouped those vessels which, 

 ■while difEering considerably in the details, 

 agree in having thickened walls, which are 

 generally perforated at the places where 

 similar vessels touch each other. The 

 thickening, and as a consequence the per- 

 forations, are of various kinds, but gener- 

 ally there is a tendency in the former to 

 the production of spiral bands; this is 

 more or less evident even when the bands 

 form a network. The transverse parti- 



the bark of an old 

 stem. Magnified 375 

 times. 



Fio. 14.— Sieve-tube 

 of Big-tree of Call- 



gantS.),te"frot tio^s, which may be horizontal or oblique, 

 are in some cases perforated with small 

 openings, in others they are almost or en- 

 tirely absorbed. The diameter of the vessels is usually 

 considerably greater than that of the surrounding cells and 

 elements of other tissues, and this alone in many cases may 

 serve to distinguish them. When young they contain pro- 

 toplasm, but as they become older this disappears, and they 

 then contain air. 



