104 THE STRUCTURE OF FLOWERS. 



of tlie insect, and so the flower prepares itself to maintain an 

 equilibrium under the tensions imposed upon it, and irregu- 

 larities are the result. Such, for example, occur in bilobed 

 calyces, as of Furze and Salvia; in the many forms of " lips," 

 or labella,* and enlarged anterior petals ; in dependent 

 stamens, as of Aconite and Epilohium angustifolium, or in the 

 more usually declinate condition, as of Dictamnus, Amaryllis, 

 etc. In these latter instances, in which the androecium bears 

 the burden, the anterior petal is either, as a rule, unaffected, 

 and shows no increase in size, or else there is a tendency to 

 atrophy, so that it is reduced in size, as are the keel petals 

 in Amherstia. It is sometimes even wanting altogether, as 

 in the Horse-chestnut.f 



* If the flower be resupinate, then it is the posterior organ which, 

 now being in the front, has become enlarged ; as in Viola and Orchis. 



t There has been more than one investigation into the causes of 

 zygomorphism (as by Vochtang, Ber. Deutsch. Bot. GesselL, iii. (1885), 

 p. 341 ; and Pringsheim's Jahrh. f. Wiss. Bot., xvii. (1886), p. 297 : also, 

 by Dr. F. Noll, Arbeit. Bot. Inst. Wiirzlurg, iii. (1887), p. 315). H. 

 Vochtung distinguishes three different sets of causes as producing 

 zygomorphism, viz. gravitation only ; gravitation acting on the consti- 

 tution of the organs ; and the constitution of the organs alone. 



An objection to gravitation pure and simple is, that all flowers would 

 be more or less subject to it, and become more or less zygomorphic 

 accordingly. It does not account for the infinite diversity in the forms 

 of zygomorphic organs ; nor for the many correlations for insect 

 fertilisation which exist between all parts of the flower. If to 

 gravitation, however, we add the weight of the insect, which simply 

 intensifies it, and couple with this the pressures exerted by the insect 

 in various directions, then we have an adequate theory, which 

 gravitation alone could not supply. When Vochtung speaks of " consti- 

 tution alone" as a cause, I presume he means hereditary effect. If so, 

 I would quite agree with him, as zygomorphic flowers now grow to be 

 such from purely hereditary influences. When, however, he would 

 attribute the form of Epilobium angustifolium to geotropism, as the 

 supposed cause of the lowermost petals bending upwards, and the 

 stamens and style downwards (see Fig. 34, p. Ill), I do not see how 



