28 BOTANY. 



sary first to make a cross-section of the stem to be studied in order to 

 determine exactly tlie position of sucli 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 pumpliin, squash, melon, and related plants the 

 bundles contain sieve-tissue on both outer and inner sides, that is on • 

 the side vvliich faces the axis of the stem as well as on tliat which 

 faces the circumference. This double nature of Ihe bundles of these 

 plants must be remembered in studying their sieve-tissue. 



(a) Make a longitudinal radial section through one of the larger 

 bundles of the stem of the pumpkin. The sieve-tissue will be distin- 

 guished by the thick-looking cross-partitions (Ibis is mainly due to 

 the adhesion of the protopliism to the walls). By adding alcohol or 

 glycerine the protoplasm of eacli cell may be contracted as in Fig. 13. 

 In some cases where the partitions are oblique the perforations may 

 be seen. 



(J) 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 sucli a specimen. 



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



VII. TracheaTy Tissue. — Here, as in the preceding, it is necessary, 

 especially in herbaceous plants, to first determine by a cross-section 

 the position of the woody bundles, as tracheary tissue is always con- 

 fined to them. 



(a) Make a thin longitudinal radial section through a bundle of the 

 stem of the Garden Balsam or Touch me-not (Impatiens). If success- 

 fully made it will show successively, passing outward, ringed, spiral, 

 reticulated, and sometimes scalariform and pitted vessels, with grada- 

 tions from one to the other, as in Fig. 15. 



(ft) Make a thin cross-section of the same and study carefully in 

 connection with the foregoing. 



(c) Make similar sections of the bundles of Indian corn. The large 

 vessels which can be seen with the naked eye in cross section are 

 pitted. 



((f) Study in lilte manner the tracheary tissue in the bundles of the 

 pumpkin-stem. Here the large pitted vessels (which are very dis- 

 tinctly visible to the naked eye) have their walls thrown into numer- 

 ous folds. 



Note.— The large pores which are so distinctly visible in oak, chestnut, hick- 

 ory, walnut, ash, and many other kinds of woods are pitted vessels like those of 

 the Indian corn and pumpkin. 



(e) Excellent scalariform vessels may be obtained from the bundles 

 of the leaf stalks of ferns, or better still from the underground stem. 



