46 HORTICULTURAL SOCIETY OF NEW YORK. 



crements which have occurred within that period. Thus an 

 average rate is easily and quickly determined. 



In attempting to determine some of the factors limiting the 

 vegetation of the arid regions of southwestern America the 

 writer has been led to a study of evaporation and its effects on 

 plant life. The remark is often made in these regions that 

 ''with water, anything whatever could be made to grow here" ; 

 a statement which is far too broad. With water properly ap- 

 plied, it is certain that a large number of plants will succeed in 

 the desert which could not otherwise live in such a climate ; but 

 all plants thus succeeding, unless it be in seasons of frequent 

 rains when evaporation is retarded, must be adapted to with- 

 stand the high evaporating power of the air. A large number 

 of the ordinary mesophilous plants of gardens are possessed 

 of this adaptation, but others are not. The native desert plants 

 usually have it to a great degree, but many of them are not so 

 fortunate in regard to the opposite power, to withstand a wet 

 soil and low evaporation rate at the same time. 



A number of plants were tested in this regard during the 

 summer just past, at the Desert Laboratory, Tucson, Arizona. 

 Narrow beds were prepared in the open ground, being only live 

 or six inches across and separated by irrigation trenches of the 

 same width and of an equal depth. The soil was kept moist 

 by lateral seepage from these trenches, which were filled once or 

 twice a day. The water stood in the trenches only a few hours 

 at a time and the soil did not at any time become water-logged. 

 It was constantly very near its optimum water content. The 

 soil was a heavy adobe clay, similar to that of the Chinese truck 

 gardens near Tucson. 



The evaporimeter used in recording the evaporating power 

 of the air for this experiment was the porous cup form devised 

 by the author and described in Publication No. 50 of the Car- 

 negie Institution. The evaporating surface is provided by a 

 cup of porous clay about five inches in length and three-quarters 

 of an inch in diameter, closed at one end and reinforced by 

 a thickened rim at the open end. The material of this cup is 

 similar to that of the ordinary Chamberland filter tubes. The 

 opening is closed by a perforated rubber stopper carrying a glass 



