LOSS OF WATER 245 



conditions as bleeding, i. e., when there is an abundant water 

 supply and when normal transpiration is checked by a humid 

 atmosphere. These hydathodes thus seem to be safety valves 

 which permit the excess turgor pressure to be reduced when it 

 gets too high. Guttation may be easily observed by covering a 

 young nasturtium (Tropxolum) or wheat plant with a bell jar and 

 watering abundantly. In a short time, drops of water will be seen 

 at the ends of the veins. These drops get larger and larger when 

 they finally run down the leaf or drop off. This is often errone- 

 ously taken for dew, but may be distinguished from it by its 

 special location. In these drops, plant disease bacteria often 

 live and from there infect the plant. In some cases, e. g., the 

 black rot of cabbage and the angular leaf spot of cotton, this is 

 the chief method of infection. Artificial guttation can be pro- 

 duced by injecting the plant with water under pressure from 

 the city water tap. When so attached to a supply of water 

 under pressure, the guttation drops are soon observed at the 



usual places. 



Normal guttation is best observed in the tropics where there is 

 an abundant supply of water in the soil and the air is nearly 

 always saturated with water vapor. In some plants, notably 

 aroids, such guttation is extremely large. A good healthy leaf of 

 Colocasia antiquorum has been known to yield over 110 c. c. of 

 water a day, the water dropping at times at the rate of over 

 100 drops a minute (Duchartre). 



The water pore or hydathode through which this water passes 

 is an aborted stoma whose guard cells no longer function, with 

 the result that it always remains open. Between this opening 

 and the tracheids of the leaf is a layer of parenchyma cells called 

 the epithem, which resembles the transfusion tissue normally sur- 

 rounding the tracheids. In some cases the epithem seems to 

 behave like a gland, actively secreting water out through the 

 opening. In other cases it is merely passive, with the result that 

 the composition of the water exuded is approximately that of the 

 soil or culture solution. In the Colocasia mentioned, Flood (1919) 

 found no epithem in the hydathodes, which may explain in part 

 the high guttation rate. 



Transpiration; Demonstration.— The losses thus far discussed 

 are minor and unimportant compared with the loss through trans- 

 piration, by which is meant the loss through evaporation from the 



