128 ELEMENTS OF STRUCTURAL BOTANY. 



leaves, containing flowers in their axils, are called 

 In the case of compound flower-clusters this term is 

 limited to the leaves on the peduncle or main stem, the 

 term bract let being then applied to those occurring on 

 the pedicels or subordinate stems. In the case of the 

 umbel and the head, it generally happens that a circle of 

 bracts surrounds the base of the cluster. They are then 

 called, collectively, an involucre, and in the case of 

 compound clusters a circle, of bractlets is called an 

 involucel. Bracts are often so minute as to be reduced 

 to mere scales. On the other hand they are occasionally 

 very conspicuous and showy, as, for instance, in the 

 four white bracts resembling a flower in the Bunchberry. 

 From our definition it will be evident, also, that the 

 spathe surrounding the spadix in Indian Turnip is merely 

 a bract. 



195. Floral symmetry. Before dealing with the mor- 

 phology of the separate leaf-forms which go to make up 

 the flower, a few words are necessary in regard to the 

 relations of the different sets of floral organs, both as to 

 number and as to position. The leaves which constitute 

 the flower are arranged about the axis either in whorls, 

 when the flowers are said to be cyclic; or in spirals, after 

 the manner of most foliage-leaves, in which case the 

 flowers are acyclic. Occasionally the outer sets (the 

 perianth) are in whorls, while the stamens are spirally 

 arranged ; then the flowers are said to be hemicydic. 

 The spiral arrangement prevails, as a rule, where the 

 floral organs are very numerous, as, for instance, in the 

 Water Lily and in Buttercup ; though Columbine, with 

 very numerous stamens, has cyclic flowers. 



