MEASUREMENT OF TENSIONS 185 



259. Bleeding Pressure, and Nectarial Excretion. The con- 

 tinued absorption of solutions by the long root -hairs sets up a great 

 osmotic pressure interior to the plasmatic membranes, and the 

 exudation pressure in these cells forces some of the fluid out of 

 the hair into the cortex at the base. Similar exudation pressure 

 in the cortex forces water into the vessels and tracheids, and this 

 process continues until the column of water thus formed mounts 

 up in the stem and reaches the apices of low, rapidly growing 

 species. Action of this character is liable to occur in all parts of 

 the plant where turgescent active tissues are in contact with xylem, 

 and is not peculiar to roots. Thus an excised stem placed in 

 water may show a bleeding pressure. The exudation of water 

 from the tips of laminae and from some water pores is due to 

 the same series of causes. The excretion of nectar from glands, 

 stigmatic surfaces, and certain water pores is due to the osmotic 

 attraction of sugars formed external to the walls of the cells lining 

 the gland. 1 



The exudation from a plant diminishes as the turgidity lessens, 

 and also with low temperatures, while the absence of oxygen and 

 the influence of anaesthetics inhibits the excretion, though the 

 cells may be turgid. Exudation pressure shows a seasonal and 

 a daily periodicity, in plants in the temperate zone. It is usually 

 greatest in the early spring before the beginning of growth, and 

 least during the period of greatest transpiration. The amount of 

 water excreted by a plant in this manner may be greater than its 

 own bulk, and it may be thrown out with a pressure exceeding 

 one atmosphere in certain instances, though usually much below 

 this. 



260. Measurement of Tension of Fluids in Body of Plant. The 

 tension of liquids in cells may be best estimated by an analysis of 

 the turgidity by means of plasmolysing agents of known isotonic 

 value (See Appendix). The tension under which liquids and gases 

 are found in the non-living elements, and in the intercellular 



1 Trelease, W. Nectar : its nature, occurrence, and uses. 1879. 



