Page:Popular Science Monthly Volume 79.djvu/549

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545
DESERT PLANTS

ures from the prototypes, that the relationship of the group is very difficult to determine.

Chief interest in the present connection lies in the fact that in the evolutionary movement the members of the group have undergone all of the specializations of the spinose forms in addition to a number of others of even more sweeping morphological importance. Stems have been reduced and branching restricted: leaves are retained by some; in others, such as the prickly pears, they appear only as rudiments dropping off before maturity, while in others, such as the great melon cacti and the sahuaro, they are not visibly represented at all. So far does the general reduction go in the Echinocacti or melon cacti that the adult plant consists of a short stem, a few inches, or at most less than two yards high, unbranched, and bearing only two types of spines which may be taken to represent the rudiments of atrophied organs, or specialized organs, largely according to the morphological prejudices of the observer. These plants represent the climax of specialization to desert conditions and the end result of the influence of aridity on the development of land vegetation.

In these succulents which constitute the highest group of desert plants, the cortex and medulla of the stems are exaggerated to an enormous extent and the greater bulk of the plant consists of a parenchymatous tissue with mucilaginous cell-contents, which gives to the

PSM V79 D549 Echinocacti and ibervillea tested for water loss rate.png

Fig. 2. A Group of Echinocacti and Ibervillea, isolated for Determination of the Rate of Water Loss. Succulents of this type have osmotic pressures of less than 12 atmospheres and absorb water only from soils containing large proportions of moisture. Some of these individuals have been without an external supply for thirty-eight months.