HETEROMOEPHIO FERTILISATION. 285 



This has been specially noticed by Hildebrand in the case of barley, 

 and Mr. Stephen Wilson states that the same thing occurs in wheat 

 and oats. Delpino remarks that in an ear of barley there are certain 

 flowers differently constructed from the rest, in which cross-fertilisation 

 is possible, and that in the oat the process varies according to the 

 weather. ' In fine warm weather the flowers open freely, and cross- 

 fertilisation is favoured ; while in cold wet weather they remain 

 closed, and self-fertilisation is inevitable. In rye, fertilisation from the 

 pollen of other flowers is provided for.* 



Certain flowers of Primrose are called pin-eyed, having a long style 

 with the rounded stigma projecting beyond the tube of the corolla, 

 and standing high above the anthers, which are situated half-way 

 down the tube ; others are called thumb-eyed, having a short style, 

 with the anthers attached at the mouth of the tube, and therefore 

 high above the stigma. These flowers occur on distinct plants. 

 Such species are dimorphic, and may be conveniently called dioeciously- 

 hermaphrodite — that is, having two kinds of hermaphrodite flowers 

 on distinct plants. EflBcient fertilisation is only attained by the 

 application of the pollen from stamens of a given length to styles of ^ 

 a corresponding length. The short styles are of the same length as 

 the short stamens, and the long styles as the long stamens, and it 

 appears that the best fertilisation and the greatest number of seeds 

 are produced by the application of the pollen of the short-styled 

 flowers to the long-styled. This is called heteromorphic fertilisation, 

 in contradistiuction to homomorphic where the pistil is fertilised by 

 the pollen of its own flower. In the Ipecacuan plant (Oephaelis 

 Ipecacuanha) dimorphic flowers occur of a similar kind. Lythrum 

 Salicaria is trimorphic ; that is, it presents three forms of flowers. 

 Each of these has stamens and pistils, each is distinct in its pistil 

 from the other two forms, and each is furnished with two sets of 

 stamens difiering from each other in appearance and function. There 

 are three lengths of stamens — long, medium, and short — but, two 

 lengths only occur in the same plant ; and there are also three lengths 

 of styles, but they are not associated with stamens of corresponding 

 length. There are then three forms of flowers — 1. With short and 

 medium stamens, and long style ; 2. With short and long stamens, 

 and medium style-; 3. With medium and long stamens, and short 

 style. The stigma is best fertilised by pollen from stamens of lengths 

 corresponding to the styles. Two of the three hermaphrodite forms 

 must co-exist, and the pollen must be conveyed reciprocally from one 

 to the other, in order that either of the two may be fuUy fertile ; , but 

 unless all three forms co-exist there will be waste of two sets of 

 stamens, and the organisation of the species as a whole ■^^ill be im- 

 perfect. On the other hand, when all three hermaphrodites co-exist, 



* See Stephen Wilson's paper in Trcms. Sot. Soc., Edin., 1874. 



