GENERAL MOEPHOLOGY OF THE PLANT 87 



the intervals of the first, the third over those of the second, 

 and so on. Here, therefore, the third pair of leaves vpill 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 and Olive orders, and is called decussation, as previously 

 noticed. In some cases the succeeding pairs or whorls are not 

 thus placed directly over the intervals of those below them, 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, suggest 

 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 simultaneously in two opposite directions ; and the 

 whorl as formed of as many spirals as there are component 

 leaves. The cases m which the decussation of the whorls is 

 not exact may generally be explained by torsion of the axis. 



The constituent leaves of ^i. whorl are usually of- the same 

 age, but not always so. In the Characese the members of the 

 whorl are developed in a definite sequence. Spurious whorls 

 may arise in certain cases where in the bud one leaf develops 

 from each of many nodes, which at that time are close together. 

 By non-development of some of the internodes, many of these 

 leaves continue in close proximity, and such clusters being 

 separated by growth of a particular internode between them, 

 the originally spiral arrangement is lost and a whorled one 

 apparently takes its place. The members of each spurioiis whorl 

 are, of course, of different ages. 



The alternation or opposition of leaves is generally constant 

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

 natural orders. Thus, the Borage order (Boraginacece) have 

 alternate leaves; the Pink order (CaryojyhyllacecB), opposite; 

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

 Leguminous order {Leguminosee), alternate ; the Eose order 

 (Bosacete), alternate, &c. While the opposition or alternation of 

 leaves may be thus shown to be constant throughout entn-e 

 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 arrangement 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 successive internodes by interruptions 



