96 THE PLUM IN KANSAS. 



Heterostyled Group. — Long styled, on which the pistil is nearly 

 twice the length of the stamens. Short styled, on which the stamens 

 are nearly twice the length of the pistil. 



Bisexual Group. — Gynodioecious, on which the flowers in mor- 

 phology consist of perfect flowers, but mostly females with anthers 

 aborted and only a few grains of pollen, and these smaller and mostly 

 aborted. Andromonoecious, on which the flowers in morphology con- 

 sist of perfect flowers, but mostly males with most of the pistils want- 

 ing or only present in a rudimentary form. 



These divisions in morjjhology and function are generally well de- 

 fined, but sometimes graduate into each other. The hermaphrodite 

 form, wliich is the only one capable of self-fertilization, is now scarcely 

 found in a wild state. Most of our cultivated varieties of P. ameri- 

 cana are transition forms of this, and are somewhat difficult to classify. 

 Some of the horticultural varieties certainly belong to the next, and a 

 few are certainly heterostyled. Dichogamous varieties are more fre- 

 quently met with in nature. The proterogynous form is easily dis- 

 tinguished by most of the pistils projecting through and above the 

 j)etals, which for a day or two remain incurved over the still immature 

 stamens, thus mechanically preventing the rij)ening of the pollen, 

 and fertilization, if accomiDlished at all, has generally been effected 

 before the petals expand. I have also noticed in intense forms of 

 this and the next a considerable difference in the time in which the 

 stigma became receptive and the pollen mature after exposure to air 

 an4 sunshine. The jjroterandrous form is also easily distinguished 

 from the fact that the pistil, before the petals expand, is found curved 

 within the corolla, the stigma being inverted and partly within the 

 calyx tube. About twenty-four to thirty-six hours after the petals 

 expand, and usually after the pollen on the anthers has all dehisced, 

 the pistil gradually assumes an upright position and becomes recep- 

 tive ; and as the stigma does not become recejitive until after an ex- 

 l^osure to the influence of light and air for at least an hour or two, 

 self-fertilization is prevented. The peculiarities of the two dichoga- 

 mous forms not only in a measure mechanically prevent self-fertiliza- 

 tion, but certainly prevent the maturation of the pollen grains and 

 stigmatic secretion at the same time, and this alone in the more in- 

 tense forms is suSicient to prevent self-fertilization. 



With respect to heterostyled plants Darwin says: "Unless it be 

 IDroved that one form is fully fertile only when it is fertilized with 

 pollen from another form, we have not complete evidence that the 

 species is heterostyled. But when i)istils and stamens differ in length 

 in two or three sets of individuals, and this is accompanied by a dif- 

 ference in the size of the pollen grains, or in the state of the stigma, 



