AMOUNT OF WATER EXUDED AND PRESSURE OF EXUDATION 259 



It is apparently owing to their position and structural peculiarities 

 that the active living cells can force water into the tracheae. Were they 

 also able to force water into the intercellular spaces, the latter would 

 become filled with fluid, as is the case when water is forced in by pressure, 

 or when internal water-glands are actively secreting (Sect. 48). The 

 pressures actually produced in the active cells are usually insufficient to 

 inject the intercellular spaces with water to any marked extent, if at all ; 

 but it has yet to be determined whether the intercellular spaces still 

 remain filled with air when a high exudation-pressure is attained. It is 

 still uncertain what part is played by the secretory activity of water- 

 glands and by other arrangements in preventing the injection of the inter- 

 cellular system with water (cf. Sects. 29 and 47). Water-glands afford 

 examples of localized secretory activity by which water may be forced 

 under pressure through inactive parenchyma cells, and hence it is not 

 surprising that occasionally the wood-parenchyma or pith may bleed 

 when a stem is cut across 1 . 



The exudation-pressure, as registered by a manometer, corresponds 

 in many plants, including several trees, only to a water-column of a few 

 centimetres in height, but in a very few cases may reach J to a atmospheres. 



Wieler (I.e., p. 122) gives a summary of the observations made by Hales, 

 Hofmeister, Horvath, Clark, and himself. The pressures vary very much. In 

 Moms albus=i-2 cm. of mercury; in Fraxinus excelsior, 2-1 cm. ; Ricinus communis, 

 33-4 cm.; Uriica dioica, 46 cm. Hales noticed a pressure of 107 cm. in Vitis 

 vim/era, and a similar pressure is shown by Cucurbita pepo. Clark gives 192 cm. 

 for Betula lenta, and Wieler observed the pressure rise above 139 cm. in 

 Betula alba. 



According to Pitra, the exudation-pressure of cut branches immersed in water 

 may be greater or smaller than that shown by the stump of the stem connected 

 with the roots. The highest pressures observed by him on cut leafy branches were : 

 Prunus cerasus, n-6 cm. ; Pinus sylvestris, 11-4 cm.; Betula, 7-5 cm. of mercury. 

 Pitra also made comparative experiments upon the bleeding pressure of the root- 

 system and of the sub-aerial organs of the same individual. 



The pressure registered by a manometer does not afford an exact 

 measure of the energy manifested by the individual active cells, although 

 it gives approximately the exudation-pressure of the plant as a whole, for to 

 produce any manometric pressure the water pumped onwards by the active 

 cells must filter through inactive cells and tissues, and thus overcome 

 a certain internal resistance. If the mercury column is artificially raised, 



paid here to the possibility of a formation of bubbles of gas, owing to processes of decomposition 

 being induced. 



1 Cf. C. Kraus, Forsch. a. d. Geb. d. Agriculturphysik, 1888, Bd. X, pp. 12, 24. 



S 2 



