[Chap. XXX ROOTS: PROCESSES AND SOIL RELATIONS 321 



natural xylem is filled with a sugar solution and the root is then immersed 

 in water, tlie sugar solution slowly rises in the attached glass tube. 



The observed phenomena may be duplicated by substituting for the 

 carrot root a long porous clay cup in the walls of which there is a 

 differentially permeable membrane of copper ferrocyanide.^ In the carrot 

 root the differentially permeable membrane is considered to be all the 

 layers of living cells between the xylem vessels and the water surround- 

 ing the root. 



This demonstration is probably analogous to the movement of water 

 from the soil through the cortex to the xylem vessels in a living root 

 under certain conditions. When the stems of some plants, growing with 

 an ample water supply, are cut off an inch or two abo\'e the soil, water 

 may exude from the cut surfaces. Under these circumstances large quan- 

 tities of sap exude from the stumps of a few kinds of plants, such as 

 birch and grape. The water may exude from these stumps against a pres- 

 sure of one or two atmospheres, which would be sufficient to push water 

 to the tops of small trees. Such pressures, however, are lacking at the 

 time of year when the water loss from leaves is greatest, and the results 

 of numerous experiments indicate that the rate of exudation is too slow 

 to account for the large volume of water that is lost in transpiration. 



Guttation. The extremely wet grass one often sees on lawns at night 

 and early morning is not always a result of the formation of dew. When 

 the humidity of the air is high and the soil is moist, each blade of grass 

 has a glistening drop of water at its tip. This loss of water in liquid fomi 

 at the ends of veins of leaves is called guttation. Early-morning golfers 

 are often annoyed by the fact that this water contains sugar, because 

 when the water evaporates from their hands and from the handles of 

 golf clubs a sticky sirup remains. 



Guttation may be demonstrated in the leaves of many kinds of plants 

 by placing bell jars over well-watered plants in pots or by attaching 

 leaves or branches to a water faucet by a hose connection. Guttation may 

 also occur in plants in greenhouses when excessive amounts of water have 

 been added ( Fig. 134 ) . The veins of the leaves are continuous with those 

 of the stems and roots, and where they end in the leaves there may be 

 an open meshwork of parenchyma cells covered by an epidermis con- 

 taining stomates or pores. Guttation is probably the result of conditions 



^ The copper ferrocyanide membrane is formed in the walls of a porous clay cup pre- 

 viously soaked in water, if the cup is filled with a solution of potassium ferrocyanide and 

 immersed in a solution of copper sulfate. 



