Page:Popular Science Monthly Volume 79.djvu/551

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547
DESERT PLANTS

forms. The latter penetrate the soil more deeply and are in constant absorbent contact with the soil. The succulents of southwestern deserts, without exception, have a wide-spreading root-system horizontally disposed immediately under the surface of the soil in a layer which is wetted by even a slight precipitation. An increase in moisture is the stimulus which starts the development of myriads of small absorbent rootlets and these have an absorbent capacity which results in the passage of a very large amount of water into the body of the jelly-like plant within a very brief period. (Fig. 3.)

As the rains come to an end the soil moisture soon dries to a limit in which the absorbent elements of the cacti may not act, these die and the plant stands or sits inert, anchored by the heavier roots in a soil with which it bears almost no important physiological relations until the coming of the next rainy season. It was to determine some of the features of the behavior of these plants during periods of extended deprivation of water that my own observations on the water-balance were begun in 1908 and are still being continued.

It is pertinent to say at this point that the halophytes, fleshy plants of seashores and saline areas, are not succulents in the present meaning of the term. These forms contain a large proportion of water, but it is held at high pressures (Cakile as high as 50 atmospheres, according to Lloyd), their transpiration rate corresponds with the proportion of water which they contain and water loss is consequently rapid, and as a further consequence they wilt quickly. An interesting capacity to vary the pressure of the sap in the absorbing organs has been found by English botanists.

The transpiration or loss of water from leaves or green organs of a plant may be roughly compared to the drying out and shrinkage of drops of wet gelatine; but with the modification that comes from the enclosure of the gelatine in small capsules arranged inside a chamber whose bounding walls are fairly water-proof, but which have ventilating openings, hundreds of them to the square millimeter. It would be as if a room were piled full of parchment bags distended with thin mucilage; the walls of the bags would undoubtedly be wet and water vapor would be constantly given off into the air-spaces at a rate very little affected by the composition of the water with which the mucilage is moistened. Furthermore, if the windows were open, the water vapor would be carried out and the total amount remaining lessened constantly.

The accentuated conditions at the Desert Laboratory have been favorable for the observation of a phase of transpiration which has been noted there for the first time. The transpiration of a leaf increases with the rising sun in the morning and the rate is accelerated until sometime in the forenoon, when, with all of the atmospheric factors at