In the semi-arid districts on the south slope of the mountains the flora consists chiefly of dry grasses, acacias, yuccas and cactuses.
White and sweet potatoes, yams, sweet and bitter yuccas, sago and okra, may also be mentioned.
Yams and sweet-potatoes, yuccas, malangas, cacao, rice - which is one of the most important foods of the people, but which is not yet widely cultivated on a profitable basis - and Indian corn, which grows everywhere and yields two crops yearly, may be mentioned also.
There is a close relation between the pollination of many yuccas and the life of a moth (Pronuba yuccasella); the flowers are open and scented at night when the female moth becomes active, first collecting a load of pollen and then depositing her eggs, generally in a different flower from that which has supplied the pollen.
The most common plants of the Mexican plateau are the agaves, yuccas and cacti, each of which is represented by a number of species.
The " ixtle " fibres shipped from Tampico and Chiapas are all obtained from the agaves and yuccas found growing wild.
The Lower Sonoran zone is noted for its cactuses, of which there is a great variety, and some of them grow to the height of trees; the mesquite is also very large, and the creosote bush, acacias, yuccas and agaves are common.
In the Lower Sonoran belt, soapweed, acacias (Palo Verde or Parkinsonia torreyana), agaves, yuccas and dasylirions, the creosote bush and mesquite tree, candle wood, and about seventy-five species of cactuses - among them omnipresent opuntiae and great columnar " Chayas " - make up a striking vegetation, which in its colours of dull grey and olive harmonizes well with the rigidity and forbidding barrenness of the plains.
In the arid portions of this and the tropic areas the indigenous plants are creosote, mesquite and alfileria bushes, desert acacias, paloverdes, alkali-heath, salt grass, agaves, yuccas (especially the Spanish-bayonet and Joshua tree) and cactuses.