108 COMPENDIUM OF GENERAL BOTANY. 



— is tlie term apj^lied to tlie above- explained process. This phe- 

 nomenon is i-eferable to living cells in general (experiments with 

 various stems by Pitra and C. Keaus). Pressurs due to bleeding 

 ^inhs 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 1 00 

 cm. high (Hales). However, as already stated, the jyositive pres- 

 sure is^ in general, very niaterially reduced during the period 

 of maximiim 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-pumj). 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 Nilgeli and Schwendener expressed 

 above. 



We must also mention the phenomenon of water-excretion., 

 observed in various herbaceous plants by ditt'erent authors.' At 

 niglit 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 a25paratus 

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

 like or brushlike at tlie jDoints referred to; above the vascular 

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

 "epithem. " Structures resembling stomata (the " water-pores "} 

 are found grouped at certain epidermal areas : they facilitate the 

 escape of water. 



Tlie 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 



' Sachs, Experimental Physiologie (1865), p. 236. Also more recently Vol- 

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



