44 THE RELATION OF DESERT PLANTS TO 



area of these leaves was determined by making photographic prints of 

 them by contact, on the developing paper known as "velox," cutting 

 out the white portion representing the leaves and calculating the area 

 of this portion from its weight and the area and weight of the sheet. 

 The area thus obtained is, of course, that of one side of the leaves only 

 and must be doubled for the total area. For a full description of this 

 method and data on the uniformity in weight of "velox" paper, see 

 Livingston (1905). 



Time was lacking for the determination of these areas at Tucson or 

 they would have been obtained without first drying the leaves. The 

 surface shrinkage, upon drying in the press, of leaves which are not 

 fleshy is, however very slight, not amounting to as much as 10 per cent 

 of the original area in the case of wheat, as the writer has had oppor- 

 tunity to observe. None of the leaves here worked with were of the 

 fleshy type, and thus the error here introduced is probably small. Also, 

 no account was taken of the area of the stems, from which a small 

 amount of evaporation must have taken place. On this point see Bur- 

 gerstein (1904, p. 27). 



In the following paragraphs will be presented the data from the 

 several experiments. These sets of data are numbered in Roman 

 numerals, merely for convenience of reference. 



Owing to the difficulty experienced in obtaining suitable pot cultures 

 of those plants which persist in vigorous vegetative condition during 

 the driest parts of the year, only three examples can be given of this 

 class. A single plant of the extremely xerophytic Euphorbia already 

 mentioned was available, and the only other plants of the hardier forms 

 which were used were two cultures of Fouquieria seedlings, two plants 

 constituting a culture, and each having at the time of the experiment 

 only four or five leaves. All of the other cultures of desert plants were 

 of annuals which appeared only about August 1 and which had prac- 

 tically all ripened their seeds and died by September 7. 



In the experiments which are to follow the plants stood either on a 

 shelf near the stationary evaporimeter already described, and were 

 thus mostly in the shade, or on the uncovered portion of the south 

 porch, about 2 meters from the wall of the building, where they had 

 direct sunshine during the day. In the latter case a special evapori- 

 meter stood near them. The evaporation data given in the different 

 experiments are from the appropriate evaporimeter. The plants were 

 taken inside during showers. 



Experiment I. The subject of this experiment was a thrifty plant 

 of Euphorbia, in flower at the time. It had been potted several weeks 

 and had apparently entirely recovered from injuries received in trans- 



