30 



BOTANY. 



outline, occur abundantly upon tlie oblique ends and radial faces of 

 the broad tubes (Fig. 19). In the Horsetails (Equi- 

 setum) and Adder-tongues (Ophioglossum) they are 

 prismatic, with numerous horizontal but not vertical 

 sieve-disks; in Brakes (Pteris) and many other ferns 

 they have pointed extremities, and are greatly elon- 

 gated, bearing the sieve-disks 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-disks, but which 

 are otherwise, including position in the stem, ex- 

 actly like the sieve-ducts of the larger species. 



Practical Studies. — As sieve-tissue is always found 

 in the woody bundles which run lengthwise through 

 the higher plants, it is necessary first to make a 

 cross-section of the stem to be studied in order to 

 determine exactly the position of such bundles. It 

 must be borne in mind that in most cases the sieve- 

 tissue is confined to the outer side of the bundle, 

 that is, to the side which faces the circumference of 

 the stem. In the pumpkin, squash, melon, and 

 related plants the bundles contain sieve-tissue on 

 both outer and inner sides, that is on the side which 

 faces the axis of the stem as well as on that which 

 faces the circumference. This double nature of the 

 bundles of these plants must be remembered in 

 studying their sieve- tissue. 

 Fig 19 —Sieve ^*) Make a longitudinal radial section through 

 tube of Big-tree one of the larger bundles of the stem of the pump- 

 quo^^glgant'elt ^^^- '^^^ sieve-tissue will be distinguished by the 

 taken from the thick-looking cross-partitions (this is mainly due to 

 stem. Magnlfled tlie adhesion of the protoplasm to the walls). By 

 375 times. adding alcohol or glycerine the protoplasm of each 



cell may be contracted as in Fig. 18. In some cases where the par- 

 titions are oblique the perforations may be seen. 



(b) Make very thin cross-sections of pumpkin-stem and examine 

 carefully for sieve-plates. Where the section is made close to a 

 plate it may be easily seen in such a specimen. 



(c) Make similar studies of the stem of Indian com. 



50. Tracheary Tissue. — Under this head are to be 

 grouped those vessels which, while differing considerably 

 in the details, agree in haying thickened walls, which are 



