Chap. IX.] DIMORPHISM AND TBIMORPHISM. 29 



this tendency is slightly disturbed by any change, the 

 vital forces gain in power. 



Reciprocal Dimorphism and Trimorphism. 



This subject may be here briefly discussed, and will 

 be found to throw some light on hybridism. Several 

 plants belonging to distinct orders present two forms, 

 which exist in about equal numbers and which differ 

 in no respect except in their reproductive organs; one 

 form having a long pistil with short stamens, the other 

 a short pistil with long stamens; the two having dif- 

 ferently sized pollen-grains. With trimorphic plants 

 there are three forms likewise differing in the lengths 

 of their pistils and stamens, in the size and colour of 

 the pollen-grains, and in some other respects; and as 

 in each of the three forms there are two sets of stamens, 

 the three forms possess altogether six sets of stamens 

 and three kinds of pistils. These organs are so pro- 

 portioned in length to each other, that half the sta- 

 mens in two of the forms stand on a level with the 

 stigma of the third form. Now I have shown, and the 

 result has been confirmed by other observers, that, 

 in order to obtain full fertility with these plants, 

 it is necessary that the stigma of the one form should 

 be fertilised by pollen taken from the stamens of cor- 

 responding height in another form. So that with di- 

 morphic species two unions, which may be called legiti- 

 mate, are fully fertile; and two, which may be called 

 illegitimate, are more or less infertile. With trimor- 

 phic species six unions are legitimate, or fully fer- 

 tile, — and twelve are illegitimate, or more or less infer- 

 tile. 



