SECTION 7.] 



THEIR ARRANGEMENT. 



71 



be made out by direct inspection. They may be indirectly ascertained, how- 

 ever, by studying the secondary spirals, as they are called, whicli usually 

 become conspicuous, at least two series of them, one 

 turning to the right and one to the left, as shown in 

 Pig. 191. For au account of the way in which the 

 character of the phyllotaxy may be deduced from th^ 

 secondary spirals, see Structural Botany, Chapter IV. 

 193. Phyllotaxy of Opposite andwhorled Leaves. 

 This is simple and comparatively uniform. The leaves 

 of each pair or whorl are placed over the intervals 

 between those of the preceding, and therefore under 

 the intervals of the pair or whorl next above. The 

 whorls or pairs alternate or cross each other, usually 

 at right angles, that is, they decussate. Opposite 

 leaves, that is, whorls of two leaves only, are far com- 

 moner than whorls of three or four or more members. 

 This arrangement in successive decussating pairs gives 

 an advantageous distribution on the stem in four verti- 

 cal ranks. Whorls of three give six vertical ranks, 

 and so on. Note that in descriptive botany leaves in 

 whorls of two are simply called opposite leaves ; and 

 that the term verticillate or ■whorled, is employed only 

 for cases of more than two, unless the latter number 

 is specified. 



194. Vernation, or Pr8Bfoliation^ the disposition 

 of the leaf-blades in the bud, 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 arrangement of the leaves of the 

 flower in the flower-bud. See, therefore, " Estivation, or Prsefloration." 



195. As to each leaf separately, it is sometimes straight and open in 

 vernation, but more commonly it is either hent, folded, or rolled up. When 

 the upper part is bent down upon the lower, as the young blade in the 

 Tulip4ree is bent upon the leafstalk, it is said to be Infexed or Reclined in 

 vernation. When folded by the midrib so that the two halves are jSlaced 

 face to face, it is Conduplicate (I'ig. 193), as in the Magnolia, the Cherry, 

 and the Oak. When folded back aud forth like the plaits of a fan, it is 



FlQ. 191. A young plant of the Houseleek, with the leaves (not yet expanded) 

 numbered, and exhibiting the 13-ranked arrangement; and showing secondary 

 spirals. > 



Pig. 192. Opposite leaves of Euonymus, or Spindle- tree, showing the successive 

 pairs crossing each other at right angles. 



