rose: new echeverla 267 



metals and an expression — involving certain of their physical 

 constants only — derived from thermodynamical principles on a 

 basis which is equivalent to the assumption that the "flow," 

 or permanent distortion, of a metal is a manifestation of an actual 

 melting produced by a stress (pressure) acting on the solid phase, 

 but not — or not to the same extent — on the liquid phase. Some 

 of the implications of this theory are discussed briefly. 



BOTANY. — A new Echeveria from Mexico. J. N. Rose. 



The botanical collections of the National Museum have all 

 been greatly increased during the last few years but especially 

 in those groups to which intensive study has been given, like the 

 ferns, succulents, and Laciniaria. In no other group has this 

 growth been so marked either in number of specimens or of new 

 species as in the Crassulaceae of Mexico. In the case of these 

 plants exhaustive and definitely directed field work has been 

 carried on for fifteen years, while the maintenance of a large 

 living collection has furnished good material for full and accurate 

 diagnoses and for the making of good herbarium specimens of 

 most of the species. 



In 1880 the Crassulaceae of Mexico were catalogued by Mr. 

 W. Botting Hemsley, who recognized 4 genera and 62 species. 

 When the writer began his work on the group the National Her- 

 barium contained representatives of all these genera and about 

 54 of the species, amounting to 86 sheets or specimens. 



In 1905 Britton and Rose revised this family for the North 

 American Flora. They recognized 17 genera and 164 species from 

 Mexico. Since then 32 species have been described, making the 

 total known from Mexico 196. 



All these genera are represented in the collections of the 

 National Herbarium and all the species with the exception of 8 

 or 10 of the older and obscure ones. The number of sheets has 

 arisen to a total of 847. In spite of the long-continued study of 

 this group new species are constantly coming to light, of which 

 the following is one. 



