20 THE STRUCTURE OF FLOWERS. 



Unstmmeteical Decrease in certain Floral Whorls. — 

 Another modifying cause of the change of symmetry id 

 the adaptation to insect or other agency for fertilisation. 

 This I believe to have played a most important part in modi- 

 fying flowers, as will be explained more fully hereafter, more 

 especially in affecting the Androecium and Gynoecium, than 

 the Perianth, as far as "number" is concerned, this latter 

 organ being altered by their agency, more especially in Form. 

 Thus, the loss of one ur more stamens is very characteristic of 

 certain groups, as in the Labiatce, when the remaining mem- 

 bers of the androecium become altered in length and position 

 so as to facihtate the intercrossing of distinct flowers. 



On the other hand, with inconspicuous and cleistogamous 

 flowers, there is a strong tendency to reduce the number of 

 stamens, as in Cbickweed to three, the allied species Stellaria 

 Holostea having ten. Similarly, in the cleistogamous flowers 

 of Violets they are sometimes reduced to three or two ; since 

 a very small amount of pollen is really quite sufficient to 

 fertilise a considerable number of ovules. 



The gynoecium has very frequently a less number of 

 carpels than the other whorls have parts. Now, the primary 

 effect of intercrossing is to enhance the size of the corolla 

 and to give a preponderance to the androecium. On the 

 other hand, one result is to check for a time the growth and 

 development of the gynoecium of most insect-visited herma- 

 phrodite flowers, i.e. to render the flower protandrous ; and 

 I strongly suspect that the generally reduced number of 

 carpels in highly differentiated flowers — as of the Gamojpetalce^ 

 in comparison with the Thalamiflorce and Calycifioroe — is cor- 

 related to the fact that they have been for many generations 

 visited by insects. This idea is supported by the fact that 

 bicarpellary genera sometimes tend to restore the ancestiul 

 number of the five carpels, as is occasionally the case in 

 Gesneria . 



