242 THE STRUCTURE OF FLOWERS. 



shaped stigmas. The two style arms, being fused together 

 and strongly papillose, are only useful for thrusting out the 

 pollen from the anther cylinder. In " double " forms the 

 corollas all become ligulate, the stamens disappear altogether, 

 and the style arms of the pistils assume the normal form 

 characteristic of the ray florets. They now set seed, so that 

 the entire capitulum is female, and forms fruit.* 



Polygamous states often occur in trees growing apparently 

 under the same conditions, and although we cannot doubt 

 that they are due to different degrees of nutrition, yet they 

 cannot be readily correlated to "visible differences in the 

 environment. Mr. Darwin thus describes the Ash : f "I 

 examined fifteen trees growing in the same field ; of these, 

 eight pi'oduced male flowers alone, and in the autumn not 

 a single seed; four produced only female flowers, which set 

 an abundance of seeds , three were hermaphrodites, and two 

 of them produced neai'ly as many seeds as the female trees, 

 whilst the third produced none, so that it was in function 

 a male. The separation of the sexes, however, is not com- 

 plete in the Ash; for the female flowers include stamens, 

 which drop off at an early period, and their anthers, which 

 never open or dehisce, generally contain pulpy matter instead 

 of pollen. On some female trees, however, I found a few 

 anthers containing pollen-grains apparently sound On the 

 male trees most of the flowers include pistils, but these 

 likewise drop off at an early period , and the ovules, which 

 ultimately abort, are very small compared with those in female 

 flowers of the same age." 



It may be added that the stamens are sometimes sub- 



* I found no difference whatever between the plants raised from 

 the larger seeds of the ray florets and the smaller ones of the disk 

 florets. They all gave rise to the " single " form of capitulum. 



t Forms of Flowers, p. 11. 



