IX VASCULAR PLANTS 207 



In accordance with the general opinions already expressed, it will 

 be natural to take first into consideration the fertile region or strobilus : 

 or in the higher plants the flower, which is held to be its outcome 

 in a more advanced state of development. These may, according to 

 a theory of sterilisation, be held to retain the primitive character of 

 fertility : it will be seen that they are conservative also in their 

 symmetry. 



It has become almost a commonplace of the elementary text-books 

 that the radial type of flower in the Angiosperms is the primitive and 

 the dorsiventral (or zygomorphic) the derivative condition. The ques- 

 tion of symmetry of the flower has been treated so lately and so well 

 by Goebel 1 that it is unnecessary here to discuss it in detail. He 

 distinguishes two cases : first, that in which the flowers are laid down 

 radially, and become dorsiventral in the course of development this 

 includes most of the dorsiventral types, and various influences may be 

 recognised as conducing to the result, such as the unequal incidence of 

 gravity and of light. Secondly, he distinguishes that type in which 

 dorsiventrality is brought about before the unfolding of the flower. In 

 this case he is of opinion 2 that we have in the position of the flower 

 an element of special importance, and the behaviour of the flower in 

 becoming dorsiventral only after unfolding must be taken as a starting- 

 point in any enquiry into this matter. Lateral flowers are in a different 

 position with regard to external forces from terminal flowers. According 

 to the sensitiveness of the former to external factors the configuration 

 of the flower will be changed more or less early. Such changes may 

 become inherited, and flowers so changed will be, of course, favoured 

 over others, and many of their parts will be aborted as useless members 

 after the introduction of dorsiventral structure. In this connection the 

 fact is of importance that in plants normally with zygomorphic flowers, 

 when a terminal flower appears, it is frequently of radial type, or is, as 

 it is termed, peloric. Goebel remarks, 3 "No doubt these wonderful forms 

 of flower exhibit a more primitive type than the dorsiventral flowers, 

 which are the normal ones in the plants in which they occur.'' Experi- 

 ment has shown in certain cases that peloria is related to intensity of 

 insolation, and thus it seems not impossible that the quality of the light- 

 ing, as well as position, may have had its influence in leading to 

 zygomorphy. But whatever the conclusion drawn from a complete 

 analysis of the causes leading to zygomorphy may be, that analysis, as 

 far as it has gone, and comparison also, point clearly to the radial type 

 of flower in Angiosperms as the primitive, and the dorsiventral as the 

 derivative. Similarly, in the case of inflorescences, which are often dorsi- 

 ventral in their development, it may be held as probable that the 

 dorsiventral inflorescences have proceeded from radial ones. 4 



1 Organography, Engl. edn., vol. i., p. 128. 2 Z.r., p. 132. 



3 L.c., p. 188. 4 Goebel, I.e., p. 138. 



