THE STEM 107 
the phloém, s and s’, shown in Fig. 121. These tracheids 
have large sunken places in their walls, called bordered pits 
(Fig. 123), closed by a very thin membrane through which 
water and dissolved food materials can more readily per- 
colate. In all other essentials, the internal structure of pine 
stems is like that of dicotyls. (See Plate 5.) 
C. Woopy Stemmep Dicotyu 
Marteriat. — Elm, basswood, mulberry, leatherwood, and pawpaw 
show the bast well; sassafras, slippery elm, and (in spring) hickory and 
willow show the cambium; grape and trumpet vine, the ducts. Some 
of the specimens used should be placed in coloring fluid from 3 to 8 hours 
before the lesson begins. The rate at which the liquid is absorbed varies 
with the kind of stem and the season. It is more rapid in spring and slower 
in winter. If a cutting stands too long in the fluid, the dye will gradually 
percolate through all parts of it; care should be taken to guard against this. 
118. The external layer. — While the primary structures, 
as shown in the last section, are essentially the same in all 
dicotyl stems, the continued yearly 
growth of perennials causes them to de- 
velop a number of secondary structures 
and variations of detail that differentiate 
them in a marked degree from soft- 
stemmed annuals. Take a piece of a 
three-year-old shoot of cherry, horse 
chestnut, or any convenient hardwood 
tree, and notice that the soft, green 
epidermis has given place to a thicker, 
harder, and usually darker colored bark. 
Notice the presence of lenticels (106) and — Fic. 124.—Part of a 
their porous, corky texture for the ad- Sous Sere eEC 
mission of air to the interior. They leaf scar; C,C, traces left 
are slightly raised above the surface of BY the broken ends of 
the bark, and are usually round, or passed from the stem in- 
P ; : to the leaf. Natural size. 
more or less elongated in different direc- 
tions, according as they are stretched vertically or hori- 
zontally by the growth of the axis. The characteristic mark- 
