154 PHYLLOTAXIS IN DIFFERENT NATURAL ORDERS, © 
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, 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 simultaneously in two opposite directions ; and the 
whorl as formed of as many spirals as there are component 
leaves. 
3. Phyllotaxis in different Natwral Orders, &c.—The alterna- 
tion or opposition of leaves is generally constant in the same 
species, and even in some cases throughout entire natural orders. 
Thus, the Borage order (Boraginaceex) have alternate leaves ; the 
Pink order (Caryophyllacex), opposite ; the Labiate order (La- 
biatx), opposite and decussate ; the Leguminous order (Legu- 
minose), alternate; the Rose order (Rosacex), 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 
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 of growth; or, if the 
whole of the internodes of a branch become non-developed, the 
leaves become tufted or fascicled (fig. 288), as already noticed. 
Generally, however, the relative position of leaves is so con- 
stant in the same species that it forms one of its characteristic 
distinctions. 
The arrangement of leaves probably influences, in some de- 
gree at least, the form of the stem and branches. Thus, a cer- 
tain amount of alternation commonly leads to a rounded form 
of stem or branch ; an opposite or whorled arrangement, to an 
angular stem or branch: for instance, the Labiate order of plants, 
in which the leaves are opposite and decussate, have commonly 
square stems and branches ; in the Neriwm Oleander, where the 
leaves on the young branches are placed in whorls of three, the 
stem has three angles ; and in the species of Galiwm and Rubia, 
which have whorled leaves, the stems and branches are always 
angular. M. Cagnat and others have also endeavoured to show 
that the arrangement of the leaves has a direct influence upon 
the forms of the wood, bark, and pith ; either upon one of these 
parts only, or sometimes upon them all; but, although some 
curious relations have been found to exist between the arrange- 
