482 * BOTANICAL GAZETTE [December 



(d) Comparative evaporation from a plant surface and from a 



free water surface 



from 



comparison 



of free water. It has been commonly held that the transpiring 

 surface of a land plant is normally unable to evaporate as great a 

 quantity of water as is evaporated from a free water surface of the 

 same area and exposed to the same external conditions. Gates 

 (7), experimenting with potted bog plants, makes the statement 

 that three to five times as much water is evaporated from a free 

 water surface as is evaporated from an equivalent area of leaf 

 surface during a hot summer day. This would seem reasonable 

 in view of the fact that the stomatal openings, being widely 



numerous 



approximate the amount 

 d be remembered in this < 



It 



of water plants are in general very thin, as is evidenced by their 



removing: them 



shows little or no cuticularization. It is probable, then, that the 



amount of cuticular transpiration from water plants is greater than 

 is popularly supposed, and that it may compensate to a greater or 

 lesser extent the insufficiency of stomatal openings. In view of 

 these facts the data of table VI show rather interesting results. 



Leaving out the case of the arrow-head for the present, it will 

 be seen that in a majority of the day readings the rate of evapora- 

 tion from the plant surfaces nearly equals the rate from the free 

 water surface, and in some cases even exceeds this rate. Thus, 

 on August 10, 13, 15, and 17, all of the day rates from the plant 



from 



on August 



14 and 16 all but one showed a greater rate, pickerel- w r eed in both 

 instances being deficient; but on August n there was only one, 

 and on August 12 there were no plant surfaces with a rate greater 

 than that from free water. The transpiration rates at night from 

 the plant surfaces are low compared with those from the free 

 water surface. The one exception is the water lily, whose tran- 

 spiration rate at night was relatively high compared with that of the 

 other plants, although in no case recorded did it equal that from th< 



