48 



PLANTS AND SUNLIGHT 



starting point. Draw a thread around the stem until the 

 two buds are joined. Set a pin at each bud. Observe that the 

 two buds are passed (not counting the last) and that the 

 thread makes one circuit of the stem. Representing the num- 

 ber of buds by a denominator, and the number of circuits 

 by a numerator, we have the fraction ^A, which expresses 

 the part of the circle that lies between any two buds. 



That is, the buds are 

 one-half of 360 de- 

 grees apart, or 180 

 degrees. Looking 

 endwise at the stem, 

 the leaves are seen 

 to be 2-ranked. Note 

 that in the apple 

 shoot (Fig. 84, right), 

 the thread makes 

 two circuits and five 

 buds are passed: 

 two-fifths represents 

 the divergence be- 

 tween the buds. The 

 leaves are 5-ranked. 

 112. Every plant 

 has its own arrange- 

 ment of leaves. For 

 opposite leaves, see 

 maple, box-elder, ash, lilac, honeysuckle, mint, fuchsia. For 

 2-ranked arrangement see all grasses, Indian corn, basswood, 

 elm. For 3-ranked arrangement see all sedges. For 5-ranked 

 (which is one of the commonest), see apple, cherry, pear, 

 peach, plum, poplar, willow. For 8-ranked, see holly, osage 

 orange. More complicated arrangements occur in bulbs, 

 house-leeks, and other condensed parts. The arrangement 

 of leaves on the stem is known as phyllotaxy (literally 



82. Rhubarb growing in the light. 



