1 40 OEGANOGRAPHY. 



any preceding leaf, but disposed in an infinite curve, and hence 

 called curviserial. The first forms of arrangement are looked 

 upon as normal ones. The latter will show the impossibility 

 of bringing organic forms and arrangements, in all cases, under 

 exact mathematical laws. 



We have thus endeavoured to show that when leaves are alter- 

 nate, the successive leaves form a spiral round the axis. The 

 spire may either turn from right to left, or from left to right. 

 In the majority of cases, the direction in both the stem and 

 branches is the same, and it is then said to be homodromoits ; but 

 instances also occasionally occur in which the direction is dif- 

 ferent, when it is called heterodromous. 



Opposite and, Verticillate Leaves. — We have already observed 

 with regard to these modifications of arrangement, that the suc- 

 cessive pairs, or whorls of leaves, as they succeed each other, 

 are not commonly inserted immediately over the preceding, but 

 that the second pair, or whorl, is placed over the intervals of the 

 first, the third over those of the second, and so on {fig. 261). 

 Here, therefore, the third pair of leaves will be directly over the 

 first, the fourth over the second, the fifth over the third, and so on. 

 This arrangement occurs in plants of the Labiate order, and is 

 called decussation, as also previously noticed. In some cases the 

 succeeding pairs, or whorls, are not thus placed directly over the 

 intervals of those below, but a little on one side, so that we shall 

 have to pass to some higher pair or whorl than the third, 

 before we arrive at one which is placed directly over the first. 

 Such arrangements, therefore, clearly show that the successive 

 pairs and whorls of leaves are arranged in a spiral manner with 

 regard to each other. Opposite leaves may be thus looked upon 

 as produced by two spirals proceeding up the stem simulta- 

 neously in two opposite directions, and the whorl as formed of 

 as many spirals as there are component leaves. 



The alternation and opposition of leaves is generally con- 

 stant in the same species, and even in some cases throughout 

 entire natural orders; thus, the Borage order {Boraginace<s), 

 have alternate leaves; the Cinchona order {Cinchonacecs,) op- 

 posite ; the Labiate order (Labiates), opposite and decussate ; the 

 Madder order (Bubiacecs), verticillate; the Leguminous order 

 {Legumhwsce), alternate ; the Rose order (EosacecB), alternate, 

 &c. While the opposition or alternation of leaves may be thus 

 shown to be constant throughout entire natural orders, yet the 

 change from one arrangement to another may be sometimes seen 

 upon the same stem, as in the common Myrtle and Snapdragon. 

 Other opposite-leaved plants also often exhibit an alternate ar- 

 rangement at the extremities of their young branches when 

 these grow very rapidly. In other cases alternate leaves may 

 become opposite, or whorled, by the non-development of the suc- 

 cessive internodes by interruptions of growth ; or, if the whole 



