656 Part IV, Chapter 6. 
of Pinus Montezumae reaches to the base of the summit rocks of the Nevado 
and mingling with the pines are oaks (Ouercus), Alnus acuminata, Salix lasio- 
lepis, Arbutus varians, while here along the borders of brooks grow Arctosta- 
phylos arguta, Clethra mexicana'). 
he pine forest which surrounds the Nevado is exactly duplicated at the base of the Vol- 
can de Colima. The forest of pines is heavy, ascending to the edge of the lava beds and 
ridges, while the ashen gray summit of the peak is bare, the tree limit being found at 7,500 feet 
(2,500 m). Naked rocks and volcanic scoria make a scene desolation unfit for the growth of 
trees species. On these lava fields are found species of Arctostaphylos and dwarf willows and 
oecasionally a Yucca, while on the topmost ridges Ribes jorullensis (= R. ciliatum) forms 
thickets with Valeriana subineisa with soft, woody stems, six to fifteen feet long, reclining on the 
shrubs summi mountain spurs are surmounted in places by thickets of Ceanothus 
azureus. — All the summit of the Nevado is covered with volcanie ashes and scoria, which do 
not offer conditions to plant life. Lupinus mexicanus (in great clumps), Pentstemon campanulatus, 
Castilleja lithospermoides, C. tenuiflora, Euphorbia campestris and Crypsinna strieta (= Epicampes 
acroura), a bunch grass, grow in the grassy openings of the mountain top. On the mountain 
erest beside the peak were found Vaccinium geminiflorum, Draba jorullensis and on the ledges a 
prostrate juniper. 
Chapter VI. North American Tropic Zone: Section of Mexico 
and Central America, 
It is extremely difficult to give the characters which distinguish the tropics 
from other adjacent parts of the earth’s surface. In general, the mean an- 
nual temperature is greater throughout the year than in other parts, but 
this alone is not sufficient to distinguish the tropics. In school texts on geo- 
graphy the statement has been made that the tropic zone lies on each side of the 
equator between the isotherms of 73° Fahrenheit (22,8° C), and the excessive 
heat and moisture of this zone produce a corresponding luxuriance of vege- 
tation. MERRIAM on the other hand limits the tropic zone by considering the 
temperature control of the distribution of plants and animals. Tropic species, 
he thinks, require a total quantity of heat of at least 26,000° F. (14,400° C.); 
and, since the tropic life region is a broad equatorial belt, it is probable that 
both its northern and southern boundaries are marked by the isotherm show- 
ing a sum of normal positive temperatures of 26,000° F. (14,4000° C.). SCHIMPER 
finds that the characterization of the tropics depends on more complex data. 
The mean annual temperature of the air oscillates between 20° and 28° and 
in comparison with northern latitudes is fairly uniform; even the difference of 
the highest and lowest temperature of the year in the equatorial zone does 
not vary much from the daily variation amounting on the average to 10— 13°C. 
often much less, for example: 5°C. The general humidity of the air cor- 
responds in general with the rainfall, being greater in the rainy season and 
1) PRINGLE, C. G.: Notes on Mexican Travel, 1. e. VII. 162, 172; KERBER, E.: see Bibliogr. 
p- 84; Gapow, Hans: Through southern Mexico 1908: 500—513 
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