DORS1VENTRAL FLOWERS 133 



be taken as a starting-point in any inquiry 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. Lateral 

 flowers may however remain radial, as we see in many Malvaceae. 



The purely mechanical explanation which was first propounded by 

 A. P. de Candolle 1 must in my opinion be entirely rejected. According 

 to him the position of the flowers has a great influence upon their relation- 

 ships of symmetry. Every flower which in nature is terminal, erect, 

 and solitary, is radial even if it belongs to a family with usually dorsiventral 

 flowers, as for example, in Asarum amongst the Aristolochiaceae ; but if 

 lateral flowers arise below a terminal flower, these will be subjected to the 

 pressure of their neighbours 2 , and will grow outwards, where the pressure 

 is less. Other factors, such as inequalities of nutrition, of air, and of light, 

 may also have an effect. When a terminal flower appears in plants 

 which normally have no terminal flower but only strongly dorsiventral 

 lateral flowers, as occasionally happens in some Labiatae, such terminal 

 flowers are radial 3 . He also points to the fact that in the changing of 

 the symmetry of the flower relationships of correlation between the several 

 parts of the flower may also come into consideration. 



Hofmeister 4 assumed a causal relationship to gravity in the dorsi- 

 ventral flowers, just as he did in the dorsiventral shoots. 



Lastly we have the recognition by Darwin of 'the supreme dominating 

 power of insects on the structure of flowers' 5 ; and then the view that the 

 stimulus which the insect exercises may influence the form of the flower 6 , 

 which is thus expressed by Henslow 7 : ' I regard this cause as issuing 

 from the insect itself ; namely the mechanical influence of its weight and 

 pressures.' How this can explain the fact that in orchids and other plants, 

 and in the dorsiventral marginal flowers above mentioned, the right 

 disposition of the flower only comes about in the unfolding I am unable to 

 see ; the whole idea is a subjective conjecture without any practical proof 

 whatever. 



We have indeed in the fundamentals of this question not reached 



1 De Candolle, The'orie elementaire de Botanique. Paris 1819. 



2 Neither here nor in other cases where this ' pressure ' is assumed as an agent modifying form is 

 any proof furnished of its existence. 



* Such ' peloria ' occur also in other plants ; see Section IV of this book. 



4 Hofmeister, Allgemeine Morphologie, p. 581. s Darwin, Forms of Flowers, p. 147. 



6 See Naegeli, Mech.-physiol. Theorie der Abstammungslehre, where the influence of the 

 scrambling of the insects upon the form of the corolla, &c. is fantasticnlly described. 



7 Henslow, Floral Structures, p. 103. 



