142 



THE LEAVES. 



whole length of the stem. In this case, the leaves are said to he 

 decussate. In other cases, as in the Pink Family, often the succes- 

 sive pairs deviate a little from this line, so 

 that we have to pass several pairs hefore we 

 reach one exactly superposed over the pair 

 we start with. This indicates a spiral ar- 

 rangement, which falls into some one of the 

 modes already illustrated in alternate leaves* 

 only that here each node bears a pair of 

 leaves. 

 251. Yerticillate or Whorled Leaves (Fig. 211) 



follow the same modes of arrangement as op- 

 Sometimes they decussate, or the leaves of one whorl 

 correspond to the intervals of that underneath, making twice as 

 many vertical ranks as there are leaves in the whorl ; 

 sometimes they wind spirally, so that each leaf of the 

 whorl belongs to as many parallel spirals, analogous 

 to the secondary spirals in the case of alternate 

 leaves. 



252. The opposition or alternation of the leaves is 

 generally constant in the same species, and often 

 through the same family ; yet both modes occasionally 

 occur on the same stem, as in the common Snap- 

 dragon and the Myrtle. All Exogens, having their 

 cotyledons opposite, necessarily commence with that 

 mode (Fig. 103-125); many retain it throughout; 

 others change to alternation, either directly in the 

 primordial leaves (Fig. Ill", 121), or at a later 

 period. In Endogens, on the contrary, the first leaves 

 are necessarily alternate (128), and it is seldom that 

 they afterwards exhibit opposite or whorled leaves. 

 The Pine in germination commences with a whorl of 

 leaves (Fig. 133, 134) ; but the subsequent ones are 

 alternate. The Pine, however (Fig. 212), and the 

 Larch, bear what are termed 212 



253. Fascicled Leaves. These are really the leaves of an axil- 

 lary bud. They remain in a tuft or cluster because the axis of 



TIG. 211. Veiticillate or whorled loaves of a Galium or Bedstraw. 



FIG 212. Piece of a branchlet of Pitch l'ine, with three leaves in a fiscicle or bundle in 

 the axil of a thin scale {a) which answers to a primary leaf The bundle is surrounded at the 

 base by a short sheath, formed of the delicate scales of the axillary bud. 



