70 CARNEGIE INSTITUTION OF WASHINGTON. 



taken up from the slopes west of the Desert Laboratorj^ and placed on 

 supports, three in the laboratory court exposed to full sunlight, and 

 three within the laboratory in diffuse light in a room with a north expo- 

 sure. The loss in weight was determined by weighing at intervals, and 

 analyses were made after periods of approximately 5 and 8| months of 

 desiccation under the two sets of conditions. 



The principal features of desiccation of the flattened joints of the 

 opuntias native to this region being known to be something different 

 from those presented by the globose echinocacti, it was deemed essen- 

 tial to follow a number of these plants through the earher stages of 

 depleted water-balance in order to compare the variations in weight 

 and water-content with those observed in Eckinocactus. The material 

 consisted of 24 turgid joints of Opuntia discata taken from two plants 

 growing near the chemical building of the Desert Laboratory on Sep- 

 tember28, 1914. The separate joints were taken from terminal portions 

 of the plant and were cut cleanly at the base in such manner as to be 

 as nearl.v equivalent as possible in evaporative capacity. Six were 

 arranged on a wooden support in an upright position in the open, 

 exposed to the full illumination, the planes of the joints being north and 

 south; six were placed near these, but in a horizontal position, being 

 turned once every week, so that the two sides were alternately upper- 

 most; six were similarly placed in an upright position in the diffuse light 

 in the middle of the largest room in the main building of the Desert 

 Laboratory, and six in a horizontal position. 



The results of the analyses are as follows: 



Prolonged confinement in diffuse light results in a decrease in den- 

 sity of sap in Echinocactus. Exposure in the open, with consequent 

 rapid loss in weight, mxay be followed by an increase or by a decrease 

 in the density of the sap. 



Decrease in the density of sap is to be attributed to a disintegration 

 of the carbohydrates, which, in Echinocactus No. 7, amounted to 13 

 per cent of the dry weight of the cortex. The destruction of material 

 was extended to include the walls of whole masses of tissue in the cortex. 



Increase in the density of the sap might result from rapid evapora- 

 tion, which altered the proportions of water and dissolved substances, 

 or by the addition of photosynthetic products. 



The proportion of reducing sugars is greatest in the peripheral tissues 

 of normal plants, in connection with the photosynthetic activity local- 

 ized here, and decreases through the cortex to the central cylinder. 

 The reduction which takes place in desiccation and starvation reverses 

 the distribution of these substances, the greatest proportion after 

 desiccation being found in the inner cortex and the total amount being 

 reduced. 



Non-reducing soluble sugars, which are present in only minute pro- 

 portions, if at all, in normal echinocacti, are noticeable constituents of 

 the sap of desiccated plants. 



