132 



RELATIONSHIPS OF SYMMETRY 



partly causal. Christian Konrad Sprengel l was the first who endea- 

 voured to establish the teleological explanation which at the present 

 time, although with a somewhat different meaning, is regarded with most 

 favour. ' As in so many other things, there are three circumstances out 

 of which one can find an explanation of the structure of flowers, and of 

 why they are regular 2 or irregular 3 . The first is the inflorescence, 

 that is to say, the method in which the flowers are arranged on the stem 

 or the branches of a plant. The second is, that the raindrops, at least 

 when the air is calm, fall perpendicularly upon the flowers. The third is 

 the intention of nature that the flowers should be fertilized by insects, in 

 connexion with which it must be remembered that the insects, whether 

 flying or moving otherwise, generally maintain an erect position.' ' Neither 

 from the side of the insect nor from that of the rain is there the slightest 

 reason why ... a flower which is strictly erect, or strictly pendant . . . 

 should not be regular . . . the insect wherever it alights upon it can 

 fertilize it ; on the other hand horizontal flowers because they have an 

 upper and an under side 4 , and the insect usually alights upon the under 

 side, and creeps in upon one of the two . . . must be irregular.' 



By the principle of selection the appearance of dorsiventrality in 

 flowers finds its ' explanation ' in the advantages which spring from it. 

 ' The zygomorphous structure of the flower allures agents for crossing and 

 excludes useless robbers of the honey. More seeds are produced by 

 crossing, and the plants produced from them are more resistent and more 

 vigorous than those which are the result of self-fertilization. The better 

 the flower-structure is adapted to agents of crossing the stronger will 

 be the progeny which will hand on the peculiarities of the best-fitted 

 individuals to descendants 5 .' If however these forms of flowers were 

 produced entirely by variation in any direction and by survival of the 

 fittest, it is difficult to see why many terminal flowers should not also have 

 become dorsiventral ; besides there are also anemophilous plants, whose 

 flowers, as those of many grasses, are dorsiventral 6 . In my opinion 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 



1 Sprengel, Das entdeckte Geheimnis der Natur, &c., Berlin, 1793, p. 37. See also Delpino, 

 Zigomorfia florale e sue cause, in Malpighia, i. p. 245 ; Robertson, Zygomorphy and its causes, 

 in Bot. Gazette, 1888. 



2 Regular = radial. 3 Irregular = dorsiventral. 



4 Sprengel, I.e. p. 42, has accurately indicated in this the essential character of a dorsiventral 

 flower the possession of an upper and an under side ; the modern expression ' zygomorphous ' is 

 based upon a subordinate feature the existence of a right and left side. 



5 Focke. 



6 The two lower lodicules are alone developed, the upper being useless has aborted and the flower 

 is undoubtedly dorsiventral. Darwin's statement in Forms of Flowers, p. 147, that dorsiveutral 

 flowers are unknown in anemophilous plants requires modification. 



