256 



THE MOVEMENTS OF WATER 



exhibited in the excretion of water by fungal hyphae. Since Wieler (I.e., p. 2) 1 

 gives a list of plants which exhibit bleeding, it is sufficient to mention that 

 in addition to the vine and maple, the following plants are suitable for the 

 study of this phenomenon: Dahlia variabilis, Impatiens snltani, Ricinus 

 communis, Calla aethiopica, Zea mays, Nicotiana tabacum. 



The power of generating an exudation-pressure cannot be an essential 

 factor in the transference of water, for it is absent from many herbs and 

 trees, and is never shown so long as transpiration is active (Sect. 33). It 

 is, however, possible that the living cells of the root may have a tendency to 

 transfer water towards the vessels in a ' vitalistic ' manner, but at present no 

 definite conclusion can be made (Sect. 36). The accumulation of sap 

 brought about by the action of the forces which tend to produce an exudation- 

 pressure is certainly of use to the plant, especially in spring-time. Thus the 

 retarded unfoldingof the buds can be hastened by forcing 

 water into cut branches 2 . This can only take place to 

 a definite extent, for beyond certain limits the forcible 

 injection of water retards growth (cf. Sect. 47). 



The apparatus given in Fig. 33 may be used to measure 

 the pressure of exudation. The stump of the stem of a 

 plant grown in earth or water is fixed by a firm india-rubber 

 connexion to a glass tube, t, to which a mercury mano- 

 meter, M, is attached by an india-rubber tube. The tube, 

 /, is filled with water and closed by an india-rubber cork 

 through which another tube g passes. The capillary end 

 of g is closed by heating, so that no air remains in the 

 apparatus. By pushing g downwards the mercury may be 

 caused more rapidly to reach the final height which the 

 exudation-pressure can support. Instead of a tube at g a glass 

 tap may be used, as in Fig. 34, and the amount of fluid ex- 

 creted can be measured directly if the escape of the displaced 

 air is possible through the cork a. By means of an air-tight 



india-rubber cork a manometer or drainage-tube may be fixed in a hole bored into 

 a tree trunk. Schwendener s used plugs with lateral holes at their apices. 



An automatic registration is possible by allowing the changes of position of 

 a burette float to be marked on a rotating drum as the escaping sap raises the 

 float upwards (Baranetzky ; cf. supra, Sect. 38). The sap may also be collected in 

 tubes arranged as in Fig. 35, so that at stated intervals the cylinder moves and 

 a new vessel comes under the drainage-tube. The rotation may be caused by an 



FIG. 



[See also Molisch, Ann. d. Jard. hot. de Buitenzorg, 2* me supplement, 1898, p. 23; Figdor, 

 Sitzungsb. d. Wien. Akad., May 20, 1898.] 



- 1 Bohm, Sitzungsb. d. Wien. Akad., 1863, Bd. XLVin, Abth. i, p. 12; Sachs, Experimental- 

 physiol., 1865, P- 242 ; Stiasburger, Bau u. Verricht. d. Leitungsbahnen, 1891, p. 843. [Exhibited 

 especially well by the flower buds of Magnolia.] 



3 Schwendener, Sitzungsb. d. Berl. Akad., 1886. p. 583. 



