52 METABOLISM 



stoppage of the outflow, frequently occur on the cut surface, due not only to 

 the activity of the plant itself, but also to the action of Bacteria. If a fresh 

 surface be exposed, a renewal of the bleeding may not infrequently be observed, 

 but this precaution has certainly not been taken in all experiments. Finally, it 

 must be remembered that sooner or later excretion from a root-stump must 

 come to an end, since the root itself dies when deprived of the nourishment 

 which it would normally receive from the leaves. 



Since, as we have seen, the duration of the bleeding and the amount of 

 sap excreted daily show very marked variations, both specific and individual, 

 it follows that the aynount of sap excreted during the entire bleeding period 

 must also vary to a very considerable extent. In the case of palms and Agave, 

 where the bleeding is both excessive and long continued, the amounts which 

 have been recorded are enormous. Thus Agave can, according to Humboldt, 

 give off in round numbers looo lit., a single axis of inflorescence of Arenga, 

 according to Semler, produces 250 lit., although Molisch (i8g8) obtained from 

 the same plant only 18-29 "i*. Equal or even greater amounts have been 

 obtained from our native trees. Wieler (1893) obtained, for example, 36 lit. 

 from a birch tree in 8 days. Finally, with reference to the pressure exerted, 

 Wieler (1892, 122) has given us a summary of a large number of determina- 

 tions which he obtained on the subject of maximum pressures. At the present 

 moment we cannot do more than quote a few selected examples from his tables. 

 Low pressures occur in herbaceous plants ; thus Petunia gives a pressure of 

 7 mm. of mercury, Chenopodium, 16 mm., Ricinus, 334 mm., Urtica dioica, 

 462 mm., the vine from 900-1100 mm., and finally the birch, 1390 mm.., 

 while Clark obtained in Betula lenta as much as 1924 mm. These heights of 

 the mercury column may be expressed in atmospheres, by saying that in 

 Ricinus the pressure is equal to half an atmosphere, and in Betula lenta to 

 two and a half atmospheres, and these appear to be the greatest pressures 

 which can be attained as a result of normal root-bleeding. Under certain con- 

 ditions, of which we shall have to speak presently, much higher pressures 

 than these may be reached. Thus Figdor (1898) has recorded in the stems 

 of certain tropical trees pressures of 6-8 atmospheres, and Bohm (1892) and 

 Molisch (1902) have obtained respectively pressures equal to 8-6 and 6-4 

 atmospheres in our native trees. 



Just as the amount of sap excreted increases gradually, so also we find 

 that the pressure does not reach a maximum all at once ; at first it gradually 

 rises and, later on, gradually falls. The periodic variations in the amount of out- 

 flow are indicative of correspondingly periodic fluctuations in pressure. In 

 addition to daily and yearly fluctuations, irregular variations have also 

 been observed. Although we must assume that the variations in the amount 

 of outflow are due to the same causes as variations in pressure, still it does not 

 follow that other and closer connexions exist between them. A large amount 

 of water may be excreted though the pressure be quite low, and conversely 

 high pressures may be accompanied by the excretion of very little water. This 

 latter condition is exemplified in the case of the specially high pressures just 

 cited (Molisch, 1902), where obviously only a few cells are concerned in the 

 excretion of water, because these cells are separated off from their surroundings 

 by layers of cells which are impermeable to water, and in such a state of affairs 

 a high pressure may be readily attained. It is not at all improbable that there 

 are individual cells in the ordinary root which allow water to filter through 

 equally energetically ; since other cells exposed to this pressure also permit water 

 to pass, what we obtain by aid of the manometer is not the maximum activity 

 of the individual cells but the resultant of secretion plus filtration. 



It is particularly striking to note that when several manometers are con- 

 nected with a stem at different levels (Brucke, 1844), the decrease in pressure 



