LESSON 10.] ARRANGEMENT OF LEAVES IN THE BUD. 



75 



191. The arrangement of opposite leaves (181) is usually very 

 simple. The second pair is placed over the intervals of the first ; 

 the third over the intervals of the second, and so on (Fig. 147) ; the 

 successive pairs thus crossing each other, 



commonly at right angles, so as to make four 

 upright rows. And whorled leaves (Fig. 148) 

 follow a similar plan. 



192. So the place of every leaf on every plant 

 is fixed beforehand by unerring mathematical 

 rule. As the stem grows on, leaf after leaf ap- 

 pears exactly in its predes- 

 tined place, producing a per- 

 fect symmetry ; a symme- 

 try which manifests itself not 



in one single monotonous 

 pattern for all plants, but in 

 a definite number of forms 

 exhibited by different spe- 

 cies, and arithmetically ex- 

 pressed by the series of frac- 

 tions, , ^, f , |, yV' 2 8 T> & c> > according as the formative energy in 

 its spiral course up the developing stem lays down at corresponding 

 intervals 2, 3, 5, 8, 13, or 21 ranks of alternate leaves. 



193. Vernation, sometimes called Prcefoliation, relates to the way 

 in which leaves are disposed in the bud (180). It comprises two 

 things ; 1st, the way in which each separate leaf is folded, coiled, or 

 packed up in the bud ; and 2d, the arrangement of the leaves in the 

 bud with respect to one another. The latter of course depends very 

 much upon the phyllotaxy, i. e. the position and order of the leaves 

 upon the stem. The same terms are used for it as for the arrange- 

 ment of the leaves of the flower in the flower-bud : so we may pass 

 them by until we come to treat of the flower in this respect. 



194. As to each leaf separately, it is sometimes straight and 

 open in vernation, but more commonly it is either bent, folded, or 

 rolled up. When the upper part is bent down upon the lower, 

 as the young blade in the Tulip-tree is bent upon the leafstalk, 

 it is said to be inflexed or reclined in vernation. When folded 



PIG. 147. Opposite leaves of the Spindle-tree or Burning-bush. 

 FIG. 148. Whorled or vorticillate leaves of Galium or Bedstraw. 



