56 PLANTS 



If the flowers of the raceme are sessile the inflorescence is a spike; 

 if they are stalked it is a true raceme. A scaly, pendulous, 

 deciduous spike is a catkin. If the older flowers of a raceme 

 rise to the level of the terminal one because of their longer stalks, 

 and thus form a flat topped cluster, we have a corymb. When 

 the rachis is much shortened and the flowers equally stalked 

 the inflorescence is an umbel and a similar condition of the 

 rachis with sessile flowers is a capitulum. 



120. That an inflorescence is made up of a system of branches 

 is further shown by the fact that each flower springs from the 

 axil of a bract, or rudimentary leaf. These are often green, 

 but sometimes scale-like or chaffy. There is frequently a 

 series of such bracts at the base of an inflorescence forming an 

 involucre. This is especially true of capitulate inflorescences. 



Structure of the Flower 



121. A complete flower has four sets of floral leaves, all more 

 or less completely transformed for special functions and bearing 

 little resemblance to the foliage leaf. The extreme diversity 

 of species with respect to the characteristics of the flower makes 

 it impossible to give any general description of it which will 

 apply to all cases. However, an ideal flower with which all 

 others may conveniently be compared may be described as 

 follows: 



122. That part of the flower stem which bears the leaves is 

 so much shortened that it forms practically a flat surface, the 

 receptacle, upon which are borne the concentric circles of floral 

 leaves. The outermost or lowest, of these circles is called the 

 calyx. It is formed of from three to five leaves, which are green 

 and enclose the other parts in the bud. The next circle within 

 this is composed of a similar number of parts, characterized by 

 some color other than green. This circle is called the corolla, 

 and corolla and calyx together are sometimes called the peri- 



