116 COMPOUND ORGANS OF PLANTS. 



sepals are equally imperceptible, such is the gradual transition 

 of one into the other. The gradual transition of the bract 

 into the sepal is well seen in composite flowers such as the 

 marigold, the involucre or calyx of which is composed of 

 numerous bracts and sepals more or less soldered together. 

 The same transition is also visible in the common hollyhock of 

 the gardens, the leaves of which approximate together, become 

 modified in size and appearance, and slide as it were insensibly 

 into a calyx. 



As flower buds are produced in the axils of bracts, and as 

 bracts are only modified leaves, it follows that the arrange- 

 ment of flower buds follows the same law as the arrangement 

 of leaf buds, the flower bud being merely the last term of 

 ramification. 



When the flower buds are lateral and the inflorescence 

 axillary the axis elongates indefinitely, and only ceases when 

 the terminal bud is suppressed or on the approach of winter. 

 When the floral axis elongates in this manner, the lower 

 flowers are the first to expand, whilst those towards its apex 

 remain closed, and the expansion is said to be centripetal or 

 from the circumference to the centre. For when a floral axis, 

 developing indefinitely, is shortened by the non-development 

 of the floral internodes, so that the flowers are brought together 

 in clusters at its summit ; the outermost flowers, which corres- 

 pond to the lowest flowers of the lengthened axis, will be the 

 first to expand, whilst the innermost flowers, which answer to 

 those- at its apex, will remain closed. The expansion of the 

 flowers will be therefore necessarily centripetal, or from the 

 circurnferenoe to the centre. 



When the flower buds are terminal, the elongation of the 

 floral axis is necessarily arrested ; it is nevertheless able to 



