INTRODUCTION. 
The district occupied by tlic Flora to which the following pages are some contributions, is 
not defined by any political or natural limits, but by an imaginary Hne extending from Aca- 
pulco north-eastwards to Durango, thence to Chihuahua, from that city to the mouth of the 
river Colorado, in the Gulf of Cahfornia, and along the western coast of Mexico to Acapulco. 
Generally speaking, it may be said that a narrow strip of fiat land runs along the whole 
coast, immediately followed by a chain of mountains, which on their eastern side join the 
table-land of Mexico, the plains of Anahuac. Such a district, situated partly within the 
boundary of the tropics, partly without it, — and, possessing lowlands, high mountains, and 
elevated plains, has a great diversity of climate,— is subject to great extremes of temperature. 
On the coast from Acapulco to Mazatlan there are the usual tropical seasons, the wet and 
the dry; the former commencing about the end of May, and ending. towards the end of 
August, or sometimes a little later : from Mazatlan northwards to the river Colorado, a 
country situated immediately without the tropics, the equinoctial seasons are less distinctly 
marked, the climate partaking more of the character of that of the temperate zone, and being 
besides very dry. In the mountains every altitude has its own range of temperature and 
moistiu'e; the western declivity however possessing generally a higher temperature and a 
greater degree of moisture than the eastern. On the higher summits the rivulets are frozen 
during the cold season, and snow often falls.* 
The climate of the table-land of Durango and Chihuahua is, like that of the greatest 
portion of the elevated plains of Mexico, dry, differing in that respect essentially from that 
of the higher regions of the New Granadlan, Ecuadorian, and Peruvian Andes, wliere mois- 
ture is abundant, and brooks, rivers, lagoons, and lakes promote a more luxuriant vegetation 
than is to be met with in Mexico at similar elevations. The extremes of heat and cold are 
unknown. Towards the end of February the night-frosts cease; spring commences, poplars 
and willows begin to get green, peaches and apricots put forth their blossoms, but the 
temperature alone, though fast increasing during April and May, is not sufficient to awaken 
* The Author did not observe any perpetual snoT\- in the tracts he traversed. 
