224 ORIGIN OF SPECIES 



special function ; but they are now known to be of the highest 

 importance for the fertilisation of the species through the 

 aid of insects, and have probably been gained through natural 

 selection. No one until lately would have imagined that in 

 dimorphic and trimorphic plants the different lengths of the 

 stamens and pistils, and their arrangement, could have been 

 of any service, but now we know this to be the case. 



In certain whole groups of plants the ovules stand erect, 

 and in others they are suspended; and within the same 

 ovarium of some few plants, one ovule holds the former and 

 a second ovule the latter position. These positions seem at 

 first purely morphological, or of no physiological significa- 

 tion; but Dr. Hooker informs me that within the same 

 ovarium, the upper ovules alone in some cases, and in other 

 cases the lower ones alone are fertilised ; and he suggests that 

 this probably depends on the direction in which the pollen- 

 tubes enter the ovarium. If so, the position of the ovules, 

 even when one is erect and the other suspended within the 

 same ovarium, would follow from the selection of any slight 

 deviations in position which favoured their fertihsation, and 

 the production of seed. 



Several plants belonging to distinct orders habitually pro- 

 duce flowers of two kinds, — the one open of the ordinary 

 structure, the other closed and imperfect. These two kinds 

 of flowers sometimes differ wonderfully in structure, yet may 

 be seen to graduate into each other on the same plant. The 

 ordinary and open flowers can be intercrossed ; and the bene- 

 fits which certainly are derived from this process are thus 

 secured. The closed and imperfect flowers are, however, 

 manifestly of high importance, as they yield with the utmost 

 safety a large stock of seed, with the expenditure of won- 

 derfully little pollen. The two kinds of flowers often differ 

 much, as just stated, in structure. The petals in the imperfect 

 flowers almost always consist of mere rudiments, and the 

 pollen-grains are reduced in diameter. In Ononis columnse 

 five of the alternate stamens are rudimentary; and in some 

 species of Viola three stamens are in this state, two retaining 

 their proper function, but being of very small size. In six 

 out of thirty of the closed flowers in an Indian violet (name 

 unknown, for the plants have never produced with me per^ 



