ix THE DEBT OF SCIENCE TO DARWIN 465 



After a considerable amount of observation and experiment, 

 he found that bees and moths visited the flowers, and that 

 their probosces became covered with pollen while sucking up 

 the nectar, and further, that the pollen of a long-stamened 

 plant would be most surely deposited on the stigma of the 

 long-styled plants, and vice versd. Now followed a long series 

 of experiments, in which cowslips were fertilised either with 

 pollen from the same kind or from a different kind of flower, 

 and the invariable result was that the crosses between the 

 two different kinds of flowers produced more good capsules, 

 and more seeds in each capsule ; and as these crosses would 

 be most frequently effected by insects, it was clear that this 

 curious arrangement directly served to increase the fertility 

 of this common plant. 



The same thing was found to occur in the primrose, and 

 in many other species of primulacese, as well as in flax (Linum 

 perenne), lungworts (Pulmonaria), and a host of other plants, 

 including the American partridge -berry (Mitchella repens). 

 These are called dimorphic heterostyled plants. 



Still more extraordinary is the case of the common loose- 

 strife (Lythrum salicaria), which has both stamens and styles 

 of three distinct lengths, each flower having two sets of 

 stamens and one style, all of different lengths, and arranged 

 in three different ways : (1) a short style, with six medium 

 and six long stamens ; (2) a medium style, with six short and 

 six long stamens ; (3) a long style, with six medium and six 

 short stamens. These flowers can be fertilised in eighteen 

 distinct ways, necessitating a vast number of experiments, 

 the result being, as in the case of the cowslip, that flowers 

 fertilised by the pollen from stamens of the same length as 

 the styles, gave on the average a larger number of capsules 

 and a very much larger number of seeds than in any other 

 case. The exact correspondence in the length of the style of 

 each form with that of the stamens in the two other forms 

 ensures that the pollen attached to any part of the body of 

 an insect shall be applied to a style of the same length on 

 another plant, and there is thus a triple chance of the maxi- 

 mum of fertility. Some other species of lythrum, of oxalis, 

 and pontederia, were also found to have three-formed stamens 

 and styles ; and in the case of the oxalis, experiments were 

 2H 



