52 



PLANT BIOLOGY 



FIG. 18. Cross section of 

 young bamboo, showing hard 

 outer rind, woody bundles, 

 scattered through the pith. 

 The center of the stem is 

 hollow. (Courtesy of New 

 York Botanical Garden.) 



as a ring in cross section), and 

 on their outer surface more fibrous 

 bark. As the season advances, 

 the activity of these cells becomes 

 less and less, and finally growth 

 ceases during the winter. 1 



Stems of plants like the corn, bam- 

 boo, and palm have no true cambium 

 layer, and therefore even in the case 

 of plants of this type that live on 

 from year to year no annual rings are 

 formed. In the growth of these stems, 

 new bundles develop in the pith be- 

 tween those already formed. 



III. THE STRUCTURE AND ADAPTATIONS OF LEAVES 



65. Leaf arrangement. 2 Along the sides of twigs leaves are 

 arranged in such a way as to secure as much light as possible with- 

 out being shaded by the leaves above them. Thus in plants like 

 the horse-chestnut, maple, and lilac, the leaves are arranged so that 

 at a given level on the twig two leaves are opposite each other, 

 while the next pair are at right angles to the first pair. This is 

 known as an opposite arrangement. The beech, elm, and rose, on 

 the other hand, have an alternate arrangement, only one leaf being 

 found at a given level on the twig. 



66. External structure of a horse-chestnut twig. Laboratory 

 Study No. 34. (Optional.) (Maple, beech, or other woody twig 

 may be used with slight verbal changes.) 



1 Sometimes trees form more than one ring during a season. 



2 Before assigning this section for study, the teacher should dem- 

 onstrate from leafy twigs (e.g. maple, horse-chestnut, lilac, elm, 

 apple) the characteristic differences between the opposite and 

 alternate arrangement of leaves. 



