STRUCTURE.] POSITION OF LEAVES. 243 



When leaves are placed in pairs on opposite sides of a stem 

 (fig. 53.), and on the same plane, they are called opposite : if 

 more than two are opposite, they then form what is called a 



fig. 53. 



whorl, or verticillus : but if they arise at regular distances 

 from each other round the stem, and not from the same plane, 

 they are then called alternate. 



In plants having Exogenous stems, the first leaves, 

 namely, those which are present in the embryo itself (coty- 

 ledons), are uniformly opposite ; but those subsequently 

 developed are either opposite, verticillate, or alternate in 

 different species : on the contrary, in Endogens, the embryo 

 leaf is either solitary, or, if there are two, they are alternate ; 

 and those subsequently developed are usually alternate also, 

 but few cases occurring in which they are opposite. Hence 

 some have formed an opinion that the normal position of the 

 leaves of Exogens is opposite, or verticillate ; and that when 

 the leaves are alternate, this arises from the extension of a 

 node ; while that of Endogens is alternate, the whorls being 

 the result of the contraction of internodes. 



But it seems more probable that the normal position of 

 all leaves is alternate, and their position upon the stem an 

 elongated spiral, as is in many cases exceedingly apparent ; for 

 instance, in the genus Pinus, in Pandanus, which is actually 

 named Screw-pine, in consequence of the resemblance its 

 shoots bear to a screw, and in the Pine-apple : the Apple, the 



