SECTIO?^ 7.] 



THEIR ARRANGEMENT. 



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

 ever, by studying the secondari/ spirals, as tlicy are called, which usually 

 become conspicuous, at least two series of them, one 

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

 Fig. 191. For an account of the way in which the 

 character of the pliyllotaxy may be deduced from the 

 secondary spirals, sec Structural ]>otauy, Chapter IV. 

 193. Phyllotaxy of Opposite andwhorled Leaves. 

 Tills 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. "VVhorls 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 verticillute or whorled, is employed only 

 for cases of more than two, unless the latter number 

 is specified. 



194. Vernation or Prgefoliation, 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 loaves in the bud with respect to one another. 

 The latter of course dei)eiids 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, " J^lstivation, or Pralloration." 



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

 vernation, but more commonly it is cither 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 hiflexcd or Reclined ia 

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

 face to face, it is Conduplicate (Fig. 193), as in the Magnolia, the Cherry, 

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



Fig. 191. A young plant of the Houseleek, witli the leaves (not yet expanded) 

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

 spirals. 



Fio. 192. Opposite leaves of Euonynuis, or Spindle-tree, showing the successive 

 pairs crossing each other at right angles. 



