276 



LECTURE XVI. 



filtration. After I had demonstrated the extraordinary capacity for filtration of the 

 cell-wall, however, the resistance to filtration had to be chiefly ascribed to the 

 protoplasmic utricle. This, supported by Naegeli's old observations on the 

 properties of the latter, has subsequently been done by Pfeffer. 



Although as a rule the forcing up of water into the stem only takes place to any 

 considerable extent in copiously branched root-stocks, it is conceivable according 

 to my explanation that here and there similar effects may also occur in parts of 

 shoots without roots. All that is necessary is that vigorously absorbing and 

 turgescent parenchyma cells force out their water into neighbouring vessels, whence 

 it can be extruded. As a matter of fact I found that cut-off" portions of the young 

 haulms of various Grasses, when stuck with the basal ends in damp sand and 

 protected from evaporation, allowed drops of water to well forth at the upper 



cut ends. According to Pitra's very ex- 

 tensive publications, leafy branches of wood 

 plants with the leaves immersed in water, 

 so that the cut end of the shoot-axis 

 projects, allow water under pressure to 

 exude from the latter; this, according to 

 the principle given, appears quite credible, 

 although I will not deny that my own 

 researches in this direction have remained 

 without result. 



The forcing up of the water out of the 

 roots into the stem was formerly considered 

 to be probably one of the causes by which, 

 in the transpiration of plants, the water 

 absorbed from the soil is despatched into 

 the leaves. Since we know, however, that 

 the root-pressure is able to lift the water 

 36-50 feet at most, the ascent of water 

 in trees 200-300 feet high cannot be ex- 

 plained in this manner ; recourse can only 

 be had to the root-pressure for the explan- 

 ation of the transpiration-current in plants 

 of a few metres high. Since I have proved, however, that the transpiration- current 

 ascends in the walls of the wood-cells as imbibed water, and that any pressure what- 

 ever from beneath is superfluous, and taking into full consideration the fact that the 

 water ^.scending by means of the root-pressure moves in the cavities of the wood, the 

 view mentioned above must be regarded as untenable ; and the fact that the suction 

 brought about by transpiration requires far more water than is supplied by the root- 

 pressure, shows that it is completely unfounded, as indeed was already to be deduced 

 from the older observations of Hofmeister ^ If the leafy top of a plant of the Gourd, 

 Tobacco, or Sunflower, is cut off above the earth and placed in water, while the 



' That far more water is disposed of in transpiration than the root pressure is able to supply 

 in an equal time, is shown more in detail by my statements, as well as the older observations of 

 others, published by Hugo de Vries in 'Ari. des hot. Inst.' (B. I. p. 288). 



Fig. Z13. 



