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 DICOTYLS 



MATERIAL. Elm, basswood, mulberry, leathcrwood, 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- 



Fio. 124. Part of a 

 young China tree shoot, 

 showing, A, lenticels; B, 

 leaf scar ; C, C, traces left 

 by the broken ends of 

 fi brovascular bundles that 

 passed from the stem in- 

 to the leaf. Natural size. 



