PHYSIOLOGY 



211 



the internal pressure has attained a certain strength, and has 

 caused the water to fill the intercellular spaces. Hydathodes are 

 mostly found on young organs and are early developed on them. 



Since the excretion of water in the liquid form can occur when 

 the conditions are unfavourable to transpiration (Lathraea) it may in 

 a sense take the place of transpiration in maintaining the current 

 from the water-absorbing organs. Its physiological significance is 

 not, however, the same as transpiration, since the expressed water 



Kirj. 195. Transverse sections of the leaf of ,S/iji i-npiUntn. The leaf above in the 

 closed state, the half leaf below expanded. U, lower surface, without stomata ; 

 ", upper surface with stomata (,S) ; C, chlorophyllous mesophyll. (x 30. After 

 KERNER VON MARILAUN.) 



always contains salts, and sometimes also organic substances in 

 solution. In fact the quantity of salts in water thus exuded is often 

 so abundant that after evaporation a slight incrustation is formed on 

 the leaves (the lime-scales on the leaves of Saxifrages) ( 32 ). 



In some instances, also, the substances in solution in the water 

 are exuded with a purpose, as in the case of the secretions of 

 the NECTARIES, of the DIGESTIVE GLANDS of insectivorous plants, 

 and of the discharge of the viscid STIGMATIC FLUID. The excreted 

 substances in these instances exert an osmotic attraction on the 

 water in the cells of the plant ; this distinguishes such cases from 

 excretion dependent simply on the internal pressure. The substances 

 excreted by some desert plants (Eeaumuria, Tamarix) are so strongly 

 hygroscopic that the leaves remain covered with numerous drops of 



