. A practical course in botany, with especial reference to its bearings on agriculture, economics, and sanitation. Botany. THE STEM 107 the phloem, 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. Woody Stemmed Dicottl Material. — Elm, basswood, mulberry, leatherwood, and pawpaw show the bast well; sassafras, sli
. A practical course in botany, with especial reference to its bearings on agriculture, economics, and sanitation. Botany. THE STEM 107 the phloem, 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. Woody Stemmed Dicottl Material. — 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 their porous, corky texture for the ad- mission of air to the interior. They are slightly raised above the surface of the bark, and are usually round, or 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-. Fig. 124
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