130 RELATIONSHIPS OF SYMMETRY 



upon the same stock flowers turned to the side and in part dorsiventral 

 which have smaller upper petals with smaller yellow spots. It is possible 

 to find in Soldanella pusilla stocks with vertically pendent flowers which 

 are radial, and others with flowers inclined obliquely downwards, the 

 corollas of which are somewhat more expanded on the under side l . It is 

 of special interest to observe in the cases that have been quoted that the 

 dorsiventrality is induced by external factors, and that these only operate 

 at a late period after the opening of the flower. 



2. In the second category of flowers the dorsiventrality is brought 

 about before the unfolding, and here, as Noll 2 was the first to show, we 

 must distinguish two groups of dorsiventral flowers according to their 

 physiological relationships : 



(a) Those which Noll has termed ' essentially zygomorphous,' charac- 

 terized by the fact that their disposition is always of a definite kind. 

 In its ' normal position ' the dorsal side of the flower is turned upwards, 

 its ventral side downwards, and the entrance is always directed away 

 from the axis of inflorescence. The flower is thus most favourably 

 placed for the visit of insects, and if it be displaced it always recovers 

 its normal position -by definite movements. Such flowers react then to 

 changes of position differently from radial flowers. 



(b] The ' unessentially zygomorphous ' flowers of Noll are, on the 

 other hand, those which stand on the margin of the inflorescences of 

 Iberis and others amongst the Cruciferae, of Heracleum, Coriandrum 

 and others. amongst Umbelliferae, of Compositae, Dipsaceae, &c. They 

 become in a morphological sense dorsiventral, because the outwardly 

 directed portion of the corolla is more strongly developed than is the 

 inner. These flowers react to changes of position in the same way as 

 do their sister-flowers which are radial, they merely serve to increase 

 the attractive apparatus for the whole inflorescence, and the dorsiven- 

 trality of the corolla stands in no direct relationship to the pollination 

 of the several flowers. That this is so is shown by the fact that in many 

 flowers the same result is arrived at although the dorsiventrality does not 

 reach the corolla, the androecium and the gynaeceum. This happens 

 in some Rubiaceae. Some years ago, during an excursion in the Ghats 

 Mountains in India, I met with shrubs on whose bright flower-clusters 

 when at considerable distance I thought I saw white butterflies. The 

 shrubs were species of Mussaenda, about the flowers of which I made 

 at the time the following note : ' On certain of the outer flowers of the 

 inflorescence one sepal is greatly enlarged, and has become leaf-like, 



1 Herbert Spencer, Principles of Biology, ii. p. 153, figures a similar case in Campanula. 

 a Noll, Uber die norinale Stellung zygomorpher Blu'ten und ihre Orientierungsbewegungen zur 

 Erreichung derselben, in Arbeiten d. bot. Instituts in Wiirzburg, iii. 



