134 PLANT PHYSIOLOGY 



nature* that low temperatures (freezing or lower) decrease 

 the sap-pressure and the sap-flow and may stop the bleeding. 

 With a rise in temperature the pressure will rise and sap- 

 flow will be resumed. Experience shows that cold at night, 

 stopping the sap-flow, and warmth by day, causing it to be 

 renewed with vigor, are most favorable to a copious yield 

 of sap of good quality. 



The means of measuring the pressure developed by the 

 absorption of water are at the best inadequate, for the 

 various forms of pressure-gauges (manometers) employed 

 cannot be made to measure the total amount of force de- 

 veloped. A manometer measures merely the net force. 

 Each individual cell which is restrained from expanding, and 

 which absorbs more water than it gives off, exerts force, 

 develops pressure. But by no means all the cells of a plant 

 develop pressure simultaneously ; some cells develop no more 

 than average pressure, and some dead parts (e. g. ducts, 

 tracheids, etc. ) cannot develop osmotic pressure under the 

 conditions ordinarily prevailing in the plant. Against these 

 less resisting cells those under pressure and seeking to ex- 

 pand, exert force. This force, being partly or wholly un- 

 resisted, expends itself, the pressing ceUs expand, the others 

 collapse. Again, water may be forced into tracheids and 

 ducts by adjacent cells and thus the pressure of the latter will 

 be reduced. If, however, tracheids and ducts become filed 

 with sap, as is the case in early spring in the sugar maple, 

 vine, etc., pressure wiU develop in these dead parts also, be- 

 cause of the force exerted by the osmotically active living cells, 

 adjacent or more or less remote. The pressure developed by 

 one group of cells may, then, expend itself wholly in some 

 other part of the body of the plant, leaving no force to be 

 exerted upon the pressure-gauge. The pressure-gauge indi- 

 cates only that amount of force due to sap-pressure which is 

 not expended in the body of the plant itself, that is, the net 

 force as it may be called, to distinguish it from the total force. 



* Bibliography in Wieler's paper above and papers by Jones and Orton, 

 Sap-presBure and flow in Sugar Maple. Ann. Report Vermont Agric. 

 Exp. Station, 1898. Morse and Wood. Studies of Maple Sap. Bulletins 

 24, 25, 32, New Hampshire Coll. Agric. Exp. Station, 1895. 



