48 MORPHOLOGY, OR COMPARATIVE ANATOMY. 



again, we usually find a similar alternation ; the leaves of the second 

 \vhorl stand over the intervals between those of the first, the 

 leaves of the third whorl standing over the leaves of the first. 



[There is reason to believe that the arrangement of alternate leaves has 

 resulted from the development of the internodes between opposite leaves ; 

 for when the latter occur at the base of a stem and alternate leaves above, 

 it will be found that, as the internodes are gradually developed, the leaves 

 always appear in succession in a spiral order ; and most frequently the 

 sixth falls over the first, that being the last out of three pairs of decus- 

 sating leaves ; or else the ninth falls over the first, that being the first 

 leaf of the fifth pair. The leaves on becoming alternate soon cease to be 

 decussating, and gradually acquire their proper angular divergence. 

 Moreover, as decussating pairs of leaves can give rise to the ordinary series 

 of fractions, f , f, T 5 4 , 5 8 i> so alternating whorls of "threes" give rise to 

 the series -I, ^ yV- Both kinds can be well studied in the Jerusalem 

 Artichoke. G. H.] 



2 



o 



3.8 7.4 



9 



Diagram illustrating the order of development of leaves when the internodes are 

 beginning to be formed, and before the proper angular divergences exist. 



Conversely, if the internodes between the component leaves of any 

 individual spiral cycle were undeveloped, while those between successive 

 cycles were lengthened, a vertidllate arrangement would result. In 

 certain plants (for example, the Myrtle, the Antirrhinum) alternate and 

 opposite leaves occur on the same stem. This is the case also in those 

 Dicotyledons where the true leaves succeeding the opposite cotyledons 

 are alternate, as in the Scarlet Bean, Mustard, &c. 



It is requisite to distinguish between simultaneous whorU, where the parts 

 composing it are developed simultaneously, and successive whorls, where 

 the parts are developed successively, but are brought together by the non- 

 development of the internodes. The arrangement of the leaves in the 

 manner above indicated is to a greit extent connected with the deposition 

 of the fibre-vascular bundles of the stem. It should, however, be stated 

 that the arrangement of the leaves on the stem is not always the same as 

 that on the branches. 



Certain terms are in common use in descriptive works to indi- 

 cate the absolute position of the leaves upon the stem. The name 

 radical leaves is applied to those, usually of larger size than the 



