70 THE WATER-BALANCE OF SUCCULENT PLANTS. 



IBERVILLEA SONOR^. 



One of the authors has already published a note upon the great value of 

 the balance of water and food-material accumulated in the tubers of Iber- 

 villea sonorce. These tubers are enlargements of the basal part of the 

 stem, and a system of thin roots springs from the tubers and penetrates 

 the soil during- the brief midsummer rainy season of Sonora. The long, 

 thin, tendril-climbing stems are sent up during the same period, but the 

 amount of material used by them is evidently much less than that taken 

 up by the roots, for each season results in an accretion to the size and 

 weight of the tuber, although some of this is lost by decay and abscission 

 in an irregular manner. Only a few of these plants have been brought 

 under observation, but the observations upon one of them have given 

 such remarkable results that they are worth recording here, although pub- 

 lished previously in part. (Carnegie Institution of Washington Publica- 

 tion No. 99, page 20, 1908.) 



A tuber weighing not more than 7 or 8 kg. was taken from the sandy 

 soil near Torres, Sonora, in February, 1902, and shipped to New York, 

 where it was soon afterwards placed on a wooden shelf of a closed museum- 

 case, exposed to diffuse light and temperatures above freezing-point in 

 winter and to 80° or 90° F. in midsummer at extreme ranges of the ther- 

 mometer. The case was quite tight, and hence the plant was not sub- 

 jected to the evaporating action of air-currents. 



The accumulated balance of food-material and water was sufficient to 

 allow the formation of short green stems every year in summer, for eight 

 successive years, from 1902 to 1909, inclusive, and the tuber weighed 4.205 

 kg. on January 14, 1910, and 3.551 kg. on July 21, 1910. The tuber now 

 appeared distinctly wrinkled and shrunken, but was still alive. About 

 half of its original weight had been lost in 8 years, indicating that the rate 

 of loss from Ibervillea must be very low, and a test was made with a 

 small specimen which had been established in a terrace near the Desert 

 Laboratory for three years. The tuber was taken up, the vine cut off 

 above, and the sparse roots taken cleanly away from the lower surface. 

 The surface was cleaned with a stiff brush and the wounds sealed with 

 grafting- wax, so that any decrease in weight might be fairly attributed 

 to transpiration. Immediately after this preparation, on October 22, 

 1909, the weight was found to be 530 grams, which was confirmed on 

 a precision-balance to within a small fraction of a gram. The tuber was 

 now placed on a wire stand, allowing free circulation of air, but in a closed 

 and shaded room. 



Table 27 gives the records for various intervals between November 6, 

 1909, and May 12, 1910. The rate of transpiration in March was nearly 

 double that of February, an increase fairly parallel to that of the echino- 

 cacti, although much less in absolute quantity. 



