MEMOIRS FROM THE DEPARTMENT OF BOTANY OF COLUMBIA UNIVERSITY 13 



This explanation seems to me rather unsatisfactory, but I am unable to give any other 

 in its place. Dickson states that he had found the same arrangement in the European 

 P. rupestris. I have confirmed Dickson's observations in both species and find that P. 

 fruticosa has the same arrangement of stamens as P. argtota, P. glandulosa and their rela- 

 tives, but the thickening of the disk is less prominent. P. rupestris belongs to the same 

 group as P. arguta, and all species of that group and P. fruticosa and a few closely 

 related Asiatic ones have the same arrangement, but as far as I know it is not found 

 elsewhere in the tribe. In the dark red-flowered Potentillae, the disk is somewhat thick- 

 ened but the arrangement of the stamens is normal. 



In the two groups with festooned stamens, the anthers ■ are also of different form 

 from those of the rest of the tribe, being flat, scarcely at all didymous, oval in outline, 

 in some species only slightly cordate at the base, with filaments attached to their backs 

 the sacs opening by a slit along the margin. The other species have as a rule anthers 

 that are more or less didymous and open by a slit which is more or less on the inside. 

 The most remarkable exception is Stellariopsis santolinoides in which the anthers are 

 decidedly didymous, each half nearly pear-shaped and opening by a subterminal pore 

 ( Plate 95). 



In most plants belonging to the tribe the filaments are slender and filiform but in 

 Horkelia they are dilated and more or less petaloid ( Plates 56-81 ). In most they are 

 broad, in some even triangular in outline, but in others, as for instance, H. tridentaia 

 (Plate 74) and congesta (Plate 76), they are i-ather narrow and lanceolate. One un- 

 described species of the subgenus Ivesia (Plate 84) has also somewhat dilated but narrow 

 filaments. 



PISTILS AND FRUIT. 



The ovary is one-ovuled and becomes an achene. In its form there is very little 

 variation, at least in the IsTorth American species. Nearly all the members of the tribe 

 have somewhat obliquely inverted pear-shaped and slightly flattened smooth and glab- 

 rous achenes. The only one that in form departs considerably from the usual type is 

 Potentilla Anserina in which the achene is much thickened at the upper end, and 

 there obtusely triangular in cross-section, and has a very thick and corky shell. In P. 

 paradoxa ( Plate 5 ) and P. NicoUetii ( Plate 6 ), the achene has a large gibbosity on 

 the inner margin. In P. Monspeliensis, P. Canadensis and P. ramulosa the achenes are 

 somewhat ribbed and in several others more or less veined. In P. sulphurea they 

 are strongly reticulated. In P. fruticosa and P. tridentaia they are hairy, in all the 

 other species glabrous. 



