108 COMPENDIUM OF GENERAL BOTANY. 



is the term applied to the above- explained process. This phe- 

 nomenon is referable to living cells in general (experiments with 

 various stems by PITRA and C. KRAUS). Pressure due to bleeding 

 sinks very materially during the vegetative period. A safe 

 maximum of this pressure (observed in a grape-vine during the 

 spring) may be represented by a column of mercury about 100 

 cm. high (HALES). However, as already stated, the positive pres- 

 sure is, in general, very materially reduced during the period 

 of maximum transpiration. If, therefore, the highest positive 

 pressure observed can raise a column of water to only 15 m., or 

 usually less say 2 m. we cannot rationally explain the rise of 

 the sap by supposing the propelling force to be at the base of the 

 stem (in the root-system) forcing the water upward several hundred 

 feet after the manner of a force-pump. This, however, does not 

 exclude the possibility that osmotic forces of lesser intensity may 

 occur at various heights in a tree and come into active play in 

 the living cells in the neighborhood of vessels and tracheids; 

 in fact, this has been proven in a number of instances. This 

 carries us back to the ideas of Nageli and Schwendener expressed 

 above. 



We must also mention the phenomenon of water -excretion, 

 observed in various herbaceous plants by different authors. 1 At 

 night during the spring when the bleeding-pressure is high and 

 transpiration low there are noticeable copious excretions of water at 

 certain areas for example, from the apices of monocotyledonous 

 leaves as well as from the serrate edges and apices of dicotyledo- 

 nous leaves. Frequently there exists a special secreting apparatus 

 placed at given points. The water-conducting vessels expand fan- 

 like or brushlike at the points referred to ; above the vascular 

 ends there is sometimes a special tissue of colorless cells, the 

 4 ' epithein. ' ' Structures resembling stomata (the ' ' water-pores ' ') 

 are found grouped at certain epidermal areas : they facilitate the 

 escape of water. 



The above-cited investigations by SCHWENDENER concerning the 

 ascent of sap (1886) give further evidence of progress toward the 

 solution of the problem under consideration, although we are far 



1 SACHS, Experimental Physiologic (1865), p. 236. Also more recently VOL- 

 KENS, tJber Wasserausscheidung, etc., Dissertation, Berlin, 1882. 



