PROBLEM 2. The Van Stems a?id Roots Flay hi Makhig Food 



is why birch bark peels into thin sheets. 

 This growth in thickness is known as sec- 

 ondary thickness or secondary growth. 

 As it continues only the younger xylem 

 and phloem cells are active. To test your 

 knowledge of stem structure and growth 

 do Exercises io and ii. In some trunks 

 the number of annual rings cannot be 

 counted because the heartwood may dis- 

 appear, leaving the tree hollow. 



Stem variations. You have studied 

 woody stems of dicots. Herbaceous 

 stems of dicots have the same kinds of 

 tissues but in different amounts. There 

 is usually more cortical tissue and less 

 cork. Some herbaceous stems make little 

 or no cork. Also in most herbaceous 

 stems there is a larger proportion of 

 pith and some have more fibers in or 

 near the phloem than is usual in woody 

 stems. The xylem and phloem with cam- 

 bium between them, along with some 

 fibers, are found in bundles located in 

 a ring around the pith. These are called 

 vascular bundles. Pith tissue extends out 

 between the bundles. 



In monocotyledons, whether they are 

 herbs, shrubs, or trees, the separate vas- 

 cular bundles are not usually arranged 

 in a ring. They are scattered through the 

 pith. You will note other differences 

 when you do Exercise 12. Monocots 

 rarely have cambium and do not, in 

 general, have secondary thickening. The 

 stem of the bamboo and the trunks of 

 many palms have the same diameter 

 along their entire length. 



Movement through xylem and phloem. 

 Xylem is continuous from the root 

 through the stem, through its branches, 

 its finer twigs, and into the veins of the 

 leaves. See Figure 185. That is, there is a 



157 



Fig. 184 The vascular bujidles in a corn stem. 

 Corn is a moJiocot. How do these vascular 

 bundles compare m position with the vascular 

 bundles i?i a young dicot stem? Where is the 

 pith? (blakiston) 



continuous passageway of xylem tubes 

 starting underground and ending among 

 the green cells of the leaf. Water from 

 the soil travels up through these tubes. 

 When there is enough water in the soil 

 there is a nearly constant stream of water, 

 containing some of the dissolved minerals, 

 through the xylem tubes. And the flow is 

 always upward. 



Sieve tubes, too, are continuous from 

 the root, through the stem and directly 

 into the various parts of the leaf. Only 



