140 



A TEXTBOOK OF BOTANY 



[Ch. IV, 5 



illustrate (Fig. 93). When the stem is not vertical, how- 

 ever, the leaf blades swing around phototropically on their 

 petioles until they face upward, in a 

 sort of mosaic, towards the light 

 (page 57), thus obscuring the op- 

 posite arrangement, which, however, 

 can always be seen where the petioles 

 join the stems. It is important to 

 remember that phyllotaxy is a matter 

 of the place of origin of leaves upon 

 stems, and has little to do with the 

 positions which the leaf blades ulti- 

 mately assume. In some kinds of 

 plants, not two, but three, or more, 

 leaves occur at each node, forming 

 a whorl, in 

 which case the 

 leaves com- 

 monly cover 

 the gaps be- 

 tween those 

 above and below, as occurs in many 

 plants of the Lily family (Fig. 94). 

 The arrangement is particularly plain 

 in the relation of the petals to the 

 sepals in most flowers. Often, how- 

 ever, it is rendered imperfect by 

 twisting of the stems, variation of 

 number of leaves in the whorls, and 

 other less obvious causes. 



When only one leaf occurs at each 

 node, they fall collectively into a 

 spiral, and the leaves are said to be 

 alternate. In the simplest case 



the successive leaves stand one-half way around the circum- 

 ference of the stem from those next above and below, thus 



Fig. 94. — The whorled 

 arrangement. 



\ 



\ 



Fig. 95. — The alternate, 

 \ spiral, arrangement. 



