CAUSES OF ARREST 59 



protection by bracts can be dispensed with, especially when, as in the 

 Compositae, there is an involucre serving as a common protective 

 apparatus to the whole of the flowers ; the bracts in such cases are 

 arrested. Plants which have umbels are particularly instructive in this 

 respect l . On the umbels of the first and second order we find commonly, 

 although not always, the bracts of the outer branches of the inflorescence 

 or of the flowers ; they form the ' involucre ' or the ' involucel ' of descriptive 

 botany. So far as I have investigated living Umbelliferae with reference 

 to this point I find that the involucre is the more likely to be present 

 the less protection otherwise the inflorescence has. If the inflorescences 

 remain for a long time within the massive vaginae of the leaves in the 

 axil of which they arise the involucre and involucel are wanting ; if that 

 is not the case then these have a function to perform and are retained 

 for protection. In illustration of this we may compare Angelica sylvestris 

 with Daucus Carota. Of course such a relationship cannot be stated 

 as a universal rule because other relationships have to be taken into 

 account. 



The arrest of the corolla in cleistogamous flowers is a characteristic 

 example of the effect of loss of function, inasmuch as the condition here 

 is a direct consequence in many plants of external influences 2 and is not 

 a reduction ensuing through a gradual loss of function of the corolla in 

 the course of generations. 



Although at present we are not able to find in the life-relationships 

 of plants satisfactory reasons for the arrests that are observable, yet more 

 accurate investigations may yet enable us to determine these. In the 

 family of the Ranunculaceae, for example, we have a very gradual 

 gradation from the carpels of the Helleboreae which bear many ovules 

 to those of the Ranunculeae which produce only one ; in the Anemoneae 

 and Clematideae the existence of the arrested ovules can be easily proved 

 by a study of the development. When now the facts are examined 

 from the biological standpoint it will be seen that the Helleboreae have 

 only few carpels in the flowers, whilst the Ranunculeae and Anemoneae 

 have many ; in other words, diminution of the number of seeds in 

 these plants lies at the bottom of the whole arrest, for in this way those 

 that are developed will be better nourished, and this result is attained 

 either by the reduction in the number of the carpels or of the ovules. 

 That this reduction takes place can be proved in the development of 

 the Anemoneae and the Clematideae, but not in the Ranunculeae. 



1 The young inflorescences of the Cruciferae conform in every respect with young umbels, the 

 internodes of the axis of inflorescence only elongate later ; it is therefore not surprising that the bracts 

 have been suppressed. " See the Fifth Section. 



