DESERT PLANTS 547 



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 horizon- 

 tally 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 pas- 

 sage 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 (CaJcile 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 Eng- 

 lish 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 ventilat- 

 ing 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 con- 

 stantly. 



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 



