98 BOTANY. 
sary 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. 
(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 (this is mainly due to 
the adhesion of the protoplasm to the walls). By adding alcohol or 
glycerine the protoplasm of cach cell may be contracted as in Fig. 18. 
In some cases where the partitions are oblique the perforations may 
be seen. 
(0) 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 the Indian corn. 
VIL Tracheary 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. 
(0) 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. 
(@) Study in like 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. 
Norg.—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, 
