22 BOTANY. 
scribed above. In Conifers the sieve-discs, which are of irregular 
outline, occur abundantly upon the oblique ends and radial faces of 
the broad tubes (Fig. 14). In the Horsetails (Equisetum) and Adder- 
tongues (Ophioglossum) 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 elongate, 
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. Tracheary Tissue-—Under this head 
are to be grouped those vessels which, 
while differing 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 
Fig. 14“Sieve-tube form a network. The transverse parti- 
of Big-tree of Cali- 
fornia (Sequoia gi- 44 1 ; 1 
gantea) taken fron tions, which may be horizontal or oblique, 
toma Macnitea Se are in some cases perforated with small 
mies 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, 
