72 



THE CARBOHYDRATE ECONOMY OF CACTI. 



used per hour by the plants or 10.24 grams of sugar for the 54 days. The 

 original 63.90 grams of dry material of the plant was then reduced by 10.24 

 grams, leaving 53.66 grams. The burning of 10.24 grams of sugar yields 

 6.14 grams of water, or results in a net loss of 4.10 grams. The plants 

 showed a total loss of 48.0 grams, hence this quantity less 4.10 grams repre- 

 sents the amount of water lost, i. e. f 43.90 grams. There were present 

 originally 139 grams of water in the plants; less 43.90 grams would leave 

 95.10 grams; to this must be added the amount of water produced by the 

 oxidation of the carbohydrates, which brings the water-content to 101.24 

 grams. From the values thus calculated (53.66 grams of dry material and 

 101.24 grams of water) there results a theoretical water-content of 65.35 per 

 cent The value actually found was 63.20 per cent. 



There are, of course, several factors which modify the calculated results. 

 Most important of these is the rate of carbon-dioxid emission. This is 



Hours 



FIG. 2. 



probably too high and would result in bringing the calculated water-content 

 to too high a value. As can be seen from figure 2, the curve of the rate of 

 carbon-dioxid emission does not decrease in a straight line, but rather 

 abruptly, so that the total amount of sugar consumed is probably calculated 

 too high. The abrupt drop in the curve of carbon-dioxid emission is not a 

 traumatic effect ; the cause has, however, as yet not been discovered. 



It is evident that these plants do not go into a condition of rest or 

 dormancy when the water-supply is greatly diminished, but continue their 

 respiratory activity at about the same rate as when supplied with abundant 

 water. This fact has already been pointed out by Richards. It would 

 seem that this is made possible by virtue of their ability to use as food 

 material not only the simpler monosaccharides which under these conditions 

 are present in reduced quantities, but also the polysaccharides. How this 

 results in the formation of pentosans in the plant will be discussed in a 

 later section. 



