64 THE WATER-BALANCE OF SUCCULENT PLANTS. 



as that plant, although No. 1 has nearly 3 times the size of No. 10. It is 

 to be noted that the smaller specimens previously exposed, when taken 

 indoors at this time, gained by the supposed absorption of the spines. 

 Some of the increased weig-ht of loss of this plant may be attributed to the 

 fact that its base was a brownish tin box and that it stood nearer a black- 

 ish stone wall than No. 9. The reflected heat from both surfaces would 

 doubtless exercise a marked effect. Similar arrangements of rock faces 

 and exposures in a natural condition must constitute an important selective 

 factor among these plants. 



On May 12, 1909, the weight was found to be 8.080 kg., including but 

 2 remaining fruits. The total estimated loss was about 4.680 kg., a rate 

 of 30 grams daily, which was slightly less than that of No. 9, but much 

 greater than the loss of plants in an inclosed room. 



The following comment upon changes in form of this plant has been 



furnished by Mrs. E. S. Spalding: 



On November 17 three intervals of two furrows each were marked — I on the side 

 away from the wall, II on the side next the wall, and III between the two on the east. 

 The plant was left in this position until January 2. During this time No. I lost 22 units, 

 II 10 units, and III 17 units. On January 2 the plant was turned around so that II 

 faced the wall and III to the west. After this II began to contract much more rapidly 

 than before. The rate of contraction for III also increased, but No. I expanded until 

 January 22, gaining 8 units, and at the end of the experiment, May 21, was only 2 units 

 smaller than when it was turned around. Moreover, tietween March 30 and April 10 it 

 expanded 6 units after having been contracting for nearly 2 months. It can not be sup- 

 posed that the plant was not losing weight all this time, and the local expansion can 

 only be accounted for by a mechanical pull due to the fact that other parts of the plant 

 were drying out more rapidly. 



The total loss of water during its exposure may be estimated at about 

 36 per cent of its total weight, or about at 40 to 45 per cent of the water 

 present at the beginning of the observation. 



The plant was now taken to the chemical laboratory and an analysis 



made of the sap from the white inner pulp, which gave the following data: 



Specific gravity of sap 1.035 



Acidity, calculated as HjSOj per 100 c.c gram . . .1064 



Total solids per 100 c.c. of juice grams. . 7.060 



Ash-content per 100 c.c. of sap grams. . 3.000 



A small plant growing undisturbed in its habitat on the bajada west of 

 the laboratory was taken up on September 9 and sent to the chemical 

 laboratory to obtain data for comparison. Occasional rains had fallen in 

 the few weeks immediately previous to this date and the water-balance of 

 the specimen may be taken to have been in the neighborhood of the maxi- 

 mum. 1.355 kg. of the white pulp was taken, from which 1,125 c.c. of 

 liquid were obtained by a Biichner press, indicating that this tissue con- 

 tained nearly nine-tenths of its weight of water, and that of the whole 

 plant about 85 per cent was water. The analysis gave the following data: 



Specific gravity of juice 1.0095 



Acidity, calculated as H2SO4 per 100 c.c. of sap gram . . .0887 



Total solids contained in 100 c.c. of sap grams. . 2.092 



Ash-content, per 100 c.c of sap gram . . .772 



