636 



PHYSIOLOGY. 



the stamens of any given flower shed their pollen on the stigma 

 of the same flower. Sprengel, however, and recently Darwin, have 

 done much to prove that though a flower may be structurally her- 

 maphrodite, it is usually functionally bisexual, and that a greater 

 number of healthy seeds are produced when a cross-fertilization 

 between the stamens of one flower and the pistil of another flower 

 of the same species is effected. Darwin even states that in those 

 cases where self-fertilization is the rule, a cross occasionally occurs. 



Heterogonous flowers. The facts just mentioned may be illustrated by 



the case of the common Primrose, the 



flowers of which are dimorphic or hetero- Fig. 614. 



styled : in some the stamens are long and 



protrude beyond the corolla; in others 



the style is long and projecting, while the 



stamens are concealed within the corolla. 



Asa Gray, in order to emphasize the fact 



that the differences just alluded to are in 



the andrcecium and gyneecium, and not 



in the floral envelopes, or in one set of 



sexual envelopes, only proposes the term 



heterogone for such flowers. The most 



complete fertility ensues, t. e. the greatest 



number of fertile seeds is formed, 



when pollen from the long stamens is 



made (by insect agency or otherwise) 



to pass, not on to the short style of the 



same flower, but on to the long style of 



another flower. Other plants, such as 



Lythrum Salicaria, are trimorpMc, having styles and stamens of three dif- 

 ferent lengths. Reciprocal fertilization is possible between any two of 



these; but the most perfect or legitimate fertilization occurs when the 



style of one flower is impregnated with pollen from a stamen of equal 



length with itself belonging to another flower. Fertilization occurring 

 between stamens and pistils of different lengths is called illegitimate. 



Mr. Darwin's experiments show that a similar difference in the degree 

 of fertility exists in the case of the illegitimate unions of heterogonous 

 flowers, as contrasted with legitimate unions between flowers of the same 

 species, as is manifest between hybrid unions between two distinct species. 

 For instance, there is the same or a similar difficulty in effecting a fertile 

 union between different sexual forms of the same species, between a short- 

 stamened and a long-styled Primrose, for instance, as there is in effecting 

 a union between two distinct species. The union may take place, but it 

 will be either infertile, or the number of seeds and the vigour of the seed- 

 lings will be diminished. Hybrids, then, may exist not only between two 

 species, but even between differently constituted individuals of the same 

 species. Sterility of hybrids, then, is an uncertain distinctive mark of 

 species. Col. Clarke has even shown that the hybrid between two genera, 

 Elisena and Ismene, is fertile. Mr. Darwin admits two subdivisions of 

 hermaphrodite plants, viz. : 1, heterostyled or ditrimorphic flowers, as in 

 the Primrose; and 2, " cleistogamic" flowers, or flowers adapted expressly 



Polyanthus : a, stamens exserted, style 

 exserted, sta- 

 3n in section.) 



excluded; 6, style exserted, sta- 

 i included. (Se 



