LOSS OF WATER FRO:\I PLANTS IN LIQUID FORM 171 



pore 



— cpitnem 



native of India, has been observed to lose as much as 100 cc. of liquid water 

 in a single night by guttation. 



Glands are found on leaves, flower parts and other organs of the plant. 

 IVIany of them represent modified epidermal hairs or other epidermal cells. 

 Certain types of glands secrete water or, more accurately, a dilute sap. These 

 have sometimes been classed as hyda- 

 thodes, but it seems better to restrict 

 this term to the t\-pe of structure pre- 

 viously described under this name. 

 The exudation of water by glands is 

 generally called secretion and is ap- 

 parently caused by forces which de- 

 velop within the gland itself, and not 

 by a pressure developed in the sap 

 of the water conducting tissue as ap- 

 pears to be true of hydathodes. Glands 

 Which secrete water or dilute saps are 



not closelv connected with xylem ele- „ , , , , • r 



, , , , J rj.. Fig. 41. Hydathode at the margin of 



ments as are the hydathodes. The ^ ^^^^^^ ,^^^ ^^ ^^^^ .^ ^^^^.^^^1 ^,.^^^ 



mechanism of this process is unknown. (semi-diagrammatic). Note termination 

 Many glands are found in plants of vessels just back of the epithem. 

 which secrete other substances as well 



as water. Among these are sugars (as in nectar), calcium salts, resins, volatile 

 oils, and enzymes. 



If the stem of an herbaceous plant be cut or broken a slow exudation of 

 sap often occurs from the ruptured stump. A similar phenomenon can often 

 be observed in the cut or broken stems or branches of woody plants, especially 

 in the spring of the year. The exudation of sap from holes bored into maple 

 trees is a good example. This phenomenon has long been called bleeding. 

 Under certain conditions large quantities of dilute sap may be lost in this 

 process. A single vigorous grape vine may lose as much as one liter of sap 

 in a day, while under favorable conditions a sugar maple tree will yield from 

 five to six liters in twenty-four hours. 



In some species bleeding appears to be due to a "root pressure" developed 

 in the xylem elements, in others to pressures developed in the phloem, and in 

 still others to locally developed pressures in other tissues. The significance 

 of the various internal pressures which develop in the conductive tissues of 

 plants will be discussed later, particularly in relation to the problems of the 

 translocation of water and solutes in plants (Chap. XV; Chap. XXVIII). 



