The plant struggle for life.
Fighting heat and drouth.
Yet it is remarkable how Nature designs and prepares the contest—the struggle for life—that is continually going on in her world. How wonderfully she arms both offence and defence! What grounds she chooses for the conflict! What stern conditions she lays down! Given a waste of sand and rock, given a heat so intense that under a summer sun the stones will blister a bare foot like hot iron, given perhaps two or three inches of rain in a twelvemonth; and what vegetation could one expect to find growing there? Obviously, none at all. But no; Nature insists that something shall fight heat and drouth even here, and so she designs strange growths that live a starved life, and bring forth after their kind with much labor. Hardiest of the hardy are these plants and just as fierce in their way as the wild-cat. You cannot touch them for the claw. They have no idea of dying without a struggle. You will find every one of them admirably fitted to endure. They are marvellous engines of resistance.
Prevention of evaporation.
Absence of large leaves.
Exhaust of moisture.
The first thing that all these plants have to fight against is heat, drouth, and the evaporation of what little moisture they may have. And here Nature has equipped them with ingenuity and cunning. Not all are designed alike, to be sure, but each after its kind is good. There are the cacti, for example, that will grow where everything else perishes. Why? For one reason because they have geometrical forms that prevent loss from evaporation by contracting a minimum surface for a given bulk of tissue.[6] There is no waste, no unnecessary exposure of surface. Then there are some members of the family like the “old man” cactus, that have thick coatings of spines and long hairy growths that prevent the evaporation of moisture by keeping off the wind. Then again the cacti have no leaves to tempt the sun. Many of the desert growths are so constructed. Even such a tree as the lluvia d’oro has needles rather than leaves, though it does put forth a row of tiny leaves near the end of the needle; and when we come to examine the ordinary trees such as the mesquite, the depua, the palo breya, the palo verde, and all the acacia family, we find they have very narrow leaves that have a fashion of hanging diagonally to the sun and thus avoiding the direct rays. Nature is determined that there shall be no unnecessary exhaust of moisture through foliage. The large-leafed bush or tree does not exist. The best shade to be found on the desert is under the mesquite, and unless it is very large, the sun falls through it easily enough.
Gums and varnishes of bushes.
As an extra precaution some shrubs are given a shellac-like sap or gum with which they varnish their leaves and make evaporation almost impossible. The ordinary greasewood is an example of this; and perhaps because of its varnish, it is, with the cacti, the hardiest of all the desert growths. It is found wherever anything living is found, and flourishes under the fiercest heat. Its leaves always look bright and have a sticky feeling about them as though recently shellacked.
The ocatilla.