432 DR. HANS GADOW ON ALTITUDE AND 
this investigation. In this I have restricted myself to Phanerogams, minus 
Grasses, and I have assumed * for the Southern third of Mexico, 2, e. from 
21 degrees North to the Isthmus railway of Tehuantepec : 
1600 species as found in the tropies. 
2500 » » in the temperate belt, up to 7000 feet. 
1300 » » thence upwards. 
200 » » at 10,000 feet level, according to Heilprin. 
Heilprin has ascended and thoroughly investigated Citlaltepetl, Popo- 
catepetl, Hztaccihuatl, and the Nevado de Toluca. I myself have ascended 
the first two (with camps at 8500 and at 12,500 feet level) and the Nevado 
de Colima ; of lower mountains the Sierra de Ajusco up to 10,000 feet, the 
Sierra Madre del Sur, above Omilteme near Chilpancingo, to about 9000 feet, 
and the Cerro de San Felipe near Oaxaca, of the same height. 
Let us start with the 10,000 feet level. Heilprin mentions 117 genera 
with 199 species. Of these, leaving a few genera unclassed, there are about 
62 Northern, 42 endemic, temperate and high-level ascendants, and 10 
Southern ascendants and Andines. 
Thence upwards we notice a rapid decline. At 12,000 feet the genera are 
reduced to 29, namely 21 Northerners, 3 endemic temperate (Echeveria, 
Arctostaphylos, Pentstemon), 4 Andines (Colobanthus, Acæna, Pernettyia, 
Alchemilla ?), and 1 Southern (Ageratum). 
At 13,000 feet the genera are reduced to 17, namely 12 Northern, 
2 endemic temperate (Echeveria, Arctostaphylos), and 3 Andines, Colobanthus 
having dropped out, and the southern Ageratum is likewise soon falling 
behind. 
At 14,000 feet the 15 genera are composed of 11 Northerners, 3 Andines 
and the Echeveria. 
Above 14,000 feet are only Arenaria, 2 Senecio, Castilleia, Echeveria, and 
Chionolena ; inall 5 genera with 6 species. Engler (op. cit. p. 223) mentions 
Alchemilla tripartita for this high level on the Orizaba mountain. Only 
Arenaria bryoides, Chionolæna lavendulacea, and Alchemilla tripartita reach 
the snow-line. 
The tree-line lies between 13,500 and 14,000 feet, but since the only 
trees (Pinus Montezuma) are scattered for the last few hundred feet, the 
tree-line does not affect the few other plants. 
Whilst at the 10,000 feet level the 62 Northern genera are still almost 
* The numbers are necessarily vague and the tropical element of the Mexican flora is 
still very imperfectly known. These circumstances may, I trust, be an excuse for those 
corrections in my calculations which future research will no doubt find necessary. The 
principles which I have tried to draw attention to will, however, not be affected. 
