78 MacDougal. — The Water-balance of Desert Plants . 
influence of seasonal factors least favourable to transpiration. Nothing may 
be safely said as to the cause of this rise. It is to be noted in this con- 
nexion, however, that the amount of dissolved solids in this plant, instead of 
being about seven or eight parts per hundred, as shown by other desiccated 
specimens, was about half this amount, not very much more than the 
proportion shown by a turgid specimen. The disintegration of certain sub- 
stances in the walls of the cells or membranes may have altered their 
permeability and permitted a more rapid water loss. 1 
The Tree Cactus Carnegie a gigantea , Britton and Rose, or Sahuaro 
forms a trunk 25 to 50 cm. in diameter with a heavy cylinder of woody 
tissue, enclosing a medulla of a diameter amounting to 8-10 cm. in some 
instances. The cortical layer of colourless cells is also very thick, the outer 
layers of this tissue being chlorophyllose. Externally the trunk presents 
a series of longitudinal folds or ridges, separated by furrows which vary in 
depth much more widely than similar structures in the various species 
of Echinocactus . The older portions of the trunk may become so distended 
as to eliminate the folding or plaiting. The trunk may reach a height of 
12-15 metres and bear many heavy branches. The total weight of the 
larger plants might be estimated at 2-4.000 kilos, 85-90 per cent, of which 
may be taken as water. The water-content may, however, undergo a wide 
variation, as is well illustrated by the following paragraphs. 
The root-system penetrates the soil more deeply than those of the 
Echinocactus , and even these organs are thick, soft, and contain a great 
amount of fluid. 
Carnegie a No. 1 was taken up May, 1908, and freed from soil and 
roots, after which it was set in a perpendicular position on a base of loosely 
piled rocks in the open. Its estimated weight was about 40 kg. November 5, 
1908, the weight was 32-518 kg. It was now brought into the laboratory 
with the Echinocacti already described. Water was lost at a rate of 62 g. 
daily during October, and at the rate of 23 g. daily during the next six 
months, which amounted to one part in 1,700 of the weight at the beginning 
of the period. The analysis of the expressed juice obtained from the 
terminal portion of the trunk gave the following results : — 
Specific gravity 1*0355 
Acidity calculated as H 2 S 0 4 per 100 c.c. of sap 0*103 
Total solids per 100 c.c. of sap .... 9*622 
Ash content of sap per 100 c.c. .... 2-75 4 
Carnegiea No. 1 a was taken up, freed from soil and roots, October 22, 
1909, when it weighed 45*325 kg. It was suitably mounted in the labora- 
tory, and lost weight at the rate of 114 g. daily during November. This 
1 The transpiratory mechanism of Echinocactus has been described by Cannon. See Biological 
Relations of Certain Cacti, American Naturalist, vol. xl, Jan. 1906, p. 27. 
