248 PROCEEDINGS OF THE AMERICAN ACADEMY 



These are the best diagnostic characters to be had for the Polemoni- 

 aceous genera with which we have here to do. One other, Lceselia, is 

 not unlikely to be found along our Mexican frontier. The genera at 

 first sight would appear to be more obviously and strictly limited than 

 they actually prove to be ; and, except for certain connecting forms, 

 their number might be properly increased by the severance of one 

 polymorphous genus into several, which, for the want of a little ex- 

 tinction, just fail to establish their characters. These plants may also 

 interest the philosophical botanist in another particular, namely, in what 

 seems to be the indications of an incipient dimorphism, discernible in 

 sundry species, but in none of them, perhaps, completely carried out 

 into reciprocally long and short filaments and style. For instance, in 

 some species of Gilia, section Leptosiphon, the style is long in some 

 individuals and short in others, while the stamens are uniform ; on the 

 other hand, at least in one species of the section Ipomopsis the stamens 

 are exserted in some individuals and included in others, with little or 

 no obvious difference in the style. In view of these facts, we may sus- 

 pect that the two sorts of style which Professor Thurber and Profes- 

 sor Torrey have detected in the genus Phlox (namely, that more than 

 half the species have a long style, so that the stigmas are often ex- 

 serted, while the rest have very short ones, bearing the stigmas low 

 down in the tube of the corolla) are somehow of dimorphic nature. 

 Yet it is only in P. subulata that I have seen both long and short 

 styles ; and here the short-styled plant has (irrespective of this charac- 

 ter) been described as a distinct species (P. nivalis, P. Hentzii), and is 

 apt to have a pair of ovules in each cell, while the long-styled P. subu- 

 lata rarely shows more than one. Moreover, in the Speciosa group 

 this character of the style really furnishes one of the most available 

 specific distinctions. "Whatever view be taken of it, the case may 

 properly be compared with that of certain species of the generally 

 dimorphic genus Primula, mentioned by Mr. Scott (in Jour. Linn. 

 Soc. 8, p. 80), which, so far as known, are either long-styled or short- 

 styled without their complementary fellow. Similarly the two species 

 of Gilia composing the group which I have named Giliandra might 

 be regarded as the long-stamened form, of which the short-stamened 

 counterpart is unknown or non-existent. A state of things which, al- 

 though singular, is intelligible upon the doctrine of the gradual evolu- 

 tion of specific and dimorphic differences. 



