Elementary botany . amount of are consequently flaccid. Astranspiration. The tubes are • , .r n .1 filled with water, and as the mghtfall comes on the airwater transpires from the leaf becomes more moist, and thesurface its movement in the tube from a to b can be measured, conditions of light are such(After Mangin.) , , . . , also that transpiration is les-sened. Root pressure, however, is still active because the soil is still these cases drops of water may be seen exuding from the margins of theleaves due to the excess of root pressure over transpiration. Were it notfor this provisio
Elementary botany . amount of are consequently flaccid. Astranspiration. The tubes are • , .r n .1 filled with water, and as the mghtfall comes on the airwater transpires from the leaf becomes more moist, and thesurface its movement in the tube from a to b can be measured, conditions of light are such(After Mangin.) , , . . , also that transpiration is les-sened. Root pressure, however, is still active because the soil is still these cases drops of water may be seen exuding from the margins of theleaves due to the excess of root pressure over transpiration. Were it notfor this provision for the escape of the excess of water raised by root pres-sure, serious injury by lesions, as a result of the great pressure, mightresult. The plant is thus to some extent a self-regulatory piece ofapparatus so far as root pressure and transpiration are concerned. 89. Injuries caused by excessive root pressure.—Some varieties of toma-toes when grown in poorly lighted and poorly ventilated greenhouses suffer. 46 PHYSIOLOGY. serious injury through lesions of the tissues. This is brought about by thecells at certain parts becoming charged so full with water through theactivity of root pressure and lessened transpiration, assisted also probablyby an accumulation of certain acids in the cell-sap which cannot be gotrid of by transpiration. Under these conditions some of the cells hereswell out, forming extensive cushions, and the cell walls become so weak-ened that they burst. It is possible to imitate the excess of root pressurein the case of some plants by connecting the stems with a system of water pressure, when very quicklythe drops of water will begin toexude from the margins of theleaves. 90. It should be stated that inreality there is no difference betweentranspiration and evaporation, if webear in mind that evaporation takesplace more slowly from living plantsthan from dead ones, or from anequal surface of water. 91. The escape of water vapor isnot the only function of
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