A varied and indeed, at times, eccentric field of study is laid open for the botanist in Mexico, for not only is there a remarkable variety of species, but their distribution is often singular. Thus the pine-tree is often found at low elevations upon the tropic slopes, much below its proper _habitat_ upon the mountain ranges; whilst palms flourish in certain places as high as 8,000 feet above sea-level; and the extraordinary cactus forms, which in Mexico are found in their greatest development, grow both on the high mountain slopes and the tropic lowland plains. Especially will the traveller in Mexico be struck by the imposing _organo_ cactus. This extraordinary growth, in form like a series of organ pipes, from which it takes its name, or like a huge branching candelabra, arising from a single stem, is a marked feature of the landscape. A few strokes of a machete, severing the stem of one of these great succulent plants, will bring down the whole structure, weighing many tons. The cactus, especially upon the high, arid deserts of the plateau, is a striking example of a plant contending with the conditions of its environment in the struggle for life. Cacti are veritable cisterns of water, stored up against long periods of absolute drought, so that they may be able to perform their function of flowering. The _organo_ and other cacti consist of great masses of juicy green cells; and to protect the scarce commodity of water which they have collected for their own use from predatory desert beasts and birds, Nature has armed them at every point with an appalling armour of thorns, or spikes, sharp as steel, and due to these matters of offence and defence the cactus is enabled to flourish in sterile places where absolutely no other vegetation could exist. Nowhere are these conditions so marked as upon the upper reaches of the high plateau of Mexico, and the variety of the cacti is most interesting. Among the cactus species are some which are of value--great value--to the human inhabitants. Chief of these is the _maguey_ (_Agave americana_), which is indeed one of the staple resources of the country, with a varied use, as described in the pages dealing with agriculture. The _nopat_, or prickly-pear, is a useful plant, yielding a succulent fruit--the _tunas_--and is also the habitat of the cochineal. [Illustration: A ROAD IN THE TEMPERATE ZONE, WITH PALMS AND VEGETATION.]