CISSUS-CARNEGIEA. 35 



thing from its tissues. With the coming- of the dry season the aridity not 

 only causes a greater water-loss in the parasite, but also makes it more 

 difficult for it to obtain water from its host. 



A series of analyses were made which showed the condition of the juice 

 of the sahuaro during- the several seasons of the year. The immense water- 

 balance of this plant made possible much greater variations than in the 

 Cissus, which probably does not show much alteration, as growth goes on 

 at all times of suitable temperature. 



A section 60 cm. in length from the apex of a stem of a sahuaro taken 

 from the slopes east of Tucson on June 15, 1909, gave an acidity of 0.186 

 gram H,SO + per 100 c.c. of sap, the total solids being 5.924 grams, of 

 which the ash amounted to 1.556 grams. A similar section was taken 

 from another plant June 28, 1909, two days after the first rain following 

 the arid foresummer. The acidity was found to be equivalent to 150 c.c. 

 H,S0 4 per 100 c.c. of sap, the total solids being 5.352 grams, of which 

 1.740 grams were ash or inorganic matter. 



The difference between the two series of results being- wide, a second 

 sample taken from another plant from the same locality on the last-named 

 date was examined. This gave an acidity of 0.181 gram H 2 S0 4 per 100 c.c. 

 of sap, the total solids being 4.512 grams and the ash 1.724 grams. These 

 proportions may be taken to represent the average conditions of plants not 

 over 5 meters in height at the end of the dry foresummer, before any 

 noticeable addition to the water-balance has resulted from the summer 

 rains. Some individual variation is to be seen in the amount of material 

 dissolved in the sap, the acidity, and the proportion of mineral salts, and 

 also in the amount of mucus present. 



A section of stem taken August 10, 1909, after the summer rains, yielded 

 sap which showed an acidity of 0.161 gram H 2 S0 4 per 100 c.c, the amount 

 of dissolved material being 3.434 grams, 1.002 grams being inorganic. 

 These may be taken as representing conditions of maximum turgidity. 



Livingston found that the sap of the sahuaro in August showed an 

 osmotic pressure of 5.54 atmospheres by the freezing-point method and 3.9 

 to 7 atmospheres by plasmolysis of strips of living tissue (Carnegie Institu- 

 tion of Washington Pub. No. 50, p. 70, 1906). The data obtained by the 

 first method are to be taken as the more reliable, as faults have been 

 recently uncovered in plasmolytic methods which would allow no depend- 

 ence to be placed on its results in connection with the present study. 



A section of a trunk of a small sahuaro taken from the slopes near the 

 Desert Laboratory, February 17, 1910, yielded a sap that showed a total 

 solid content of 3.056 gram per 100 c.c, of which the ash or inorganic 

 material was 0.992 gram. The average of three freezing-point determi- 

 nations indicated an osmotic activity of 6.78 atmospheres at 25 C. It is to 

 be noted that these results are near those obtained in August, 1909, and 

 represent the conditions in the sahuaro at the time when most of the prep- 

 arations of xeno-parasites were made. 



