THE INFLORESCENCE AND THE FLOWER 113 



circumference. This is known as a capitulum. It is 

 generally surrounded by a number of bracts, which form 

 what is called an involucre. 



In its most primitive form the flower probably con- 

 sisted of an axis on which the sporangia were borne, and 

 the latter were of only one kind. In many cases, most 

 likely arising later, we find this axis bearing two sporan- 

 gial series of outgrowths, one for the production of each 

 kind of spore, and this arrangement gradually became 

 widespread, the large spores being developed in the 

 series nearest the apex and the small ones in the lower 

 of the two. As each series stood in a circle round the 

 axis, all its parts arising at the same node, they received 

 the name of whorls. 



In the more perfect forms of flower which have been 

 developed as time has gone on (Fig. 45) we find, besides 

 these two whorls of spore-bearing leaves, two other 

 whorls growing below them. These two constitute the 

 perianth of the flower. A leaf lower down still, in the 

 axil of which the flower arises, is known as a bract. 



The perianth of the flower is formed then by two 

 series or whorls of leaves. The outer ones are green 

 and often sturdy in their texture, and they protect 

 the young flower while it is in 

 the condition of a bud. They 

 are known as sepals, and the 

 collection of them forms the 

 calyx. The inner whorl is 

 usually made up of highly 

 coloured leaves which serve to 

 make the flower conspicuous. 

 They are called petals, and the FlG ' 45 ' ' 

 collection is termed the corolla. 

 These whorls frequently have their sepals or petals joined 

 together for part or even the whole of their length. 



The whorls which bear the sporangia are distinguished 



