ORGANS OF NUTRITION. 143 



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. Here, therefore, 

 the third pair of leaves will be directly over the first, the 

 fourth over the second, the fifth over the third, &c. This ar- 

 rangement occurs in plants of the Mint family, and is called 

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

 succeeding pairs, or whorls, are not thus placed directly OA-er 

 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 an'ive at one which is placed directly over the first. 

 Such ai-rangements 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 is 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 families of plants; thus, the Borage tribe (Boraginecs) have 

 alternate leaves; the Peruvian Bark tribe {Cinchona cece), oppo- 

 site; the Mint tribe {Labiates), opposite and decussate ; the 

 Madder tribe (Rubiucece), verticillate ; the Pea tribe {Legumi- 

 noscB), alternate ; the Kose tribe (Rosacece), alternate; &c. 

 While the opposition or alternation of leaves may be thus 

 shown to be constant throughout entire famihes, yet the change 

 from one arrangement to another may be sometimes seen upon 

 the same stem, as in the common Myrtle, in Snapdragon, &c. 

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

 rangement at the extremities of their young branches when 

 they grow very rapidly. In other cases, alternate leaves may 

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

 successive intemodes by interruptions of growth; or, if the 

 whole of the intemodes of a branch become non-developed, the 

 leaves become tufted or fascicled, as already noticed. As a 

 general rule however, the relative position of leaves is suffi- 

 ciently constant in the same species as to form one of its 

 characteristic distinctions.. 



The arrangement of leaves probably influences, in some 

 degree at least, the form of the stem and branches. Thus, a 

 certain amount of alternation commonly leads to a rounded 

 form of stem: an opposite, or whorled arrangement, to an 

 angular stem; for instance, the ]\Iint tribe of plants, which have 

 opposite and decussate leaves, have square stems ; the Nerium 

 Oleander, where the leaves on the young branches are placed in 

 whorls of three, the stem has three angles; and in the Madder 

 tribe of plants, which have whorled leaves, the stems are always 

 angular. M. Cagnat has also endeavoured to show that the 



