HYBRIDISM 2S7 



gether by some common but unknown bond, which is essentially 

 related to the principle of life; this principle, according to Mr. 

 Herbert Spencer, being that life depends on, or consists in, the 

 incessant action and reaction of various forces which, as through- 

 out nature, are always tending toward an equilibrium; and when 

 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 dis- 

 tinct 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 differently sized pollen- 

 grains. With trimorphic plants there are three forms likewise 

 differing in the lengths of their pistils and stamens, in the size 

 and color 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 proportioned in length to each other 

 that half the stamens 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 fertilized by pollen taken from the stamens 

 of corresponding height in another form. So that with dimorphic 

 species two unions, which may be called legitimate, are fully 

 fertile; and two, which may be called illegitimate, are more or 

 less infertile. With trimorphic species six unions are legitimate, or 

 fully fertile, and twelve are illegitimate, or more or less infertile. 



The infertility which may be observed in various dimorphic 

 and trimorphic plants, when they are illegitimately fertilized, that 

 is, by pollen taken from stamens not corresponding in height with 

 the pistil, differs much in degree, up to absolute and utter sterility; 

 just in the same manner as occurs in crossing distinct species. As 

 the degree of sterility in the latter case depends in an eminent 

 degree on the conditions of life being more or less favorable, so I 

 have found it with illegitimate unions. It is well known that if 

 pollen of a distinct species be placed on the stigma of a flower, 

 and its own pollen be afterward, even after a considerable interval 

 of time, placed on the same stigma, its action is so strongly pre- 

 potent that it generally annihilates the effect of the foreign pollen; 



