34 



PHYSIOLOG V. 



plants in the open niav be used if it is |")referi"c'l, Init plants <2;rown in pots 

 are also vcrv serviceable, and one ma\' itse a ])ottcd begonia or balsam, the 

 latter being especialb' useful. The plants are tisually con\"enient to obtain 

 from the greenhouses, to illustrate this phenomenon. 

 The stem is cut off rather close to the .soil and a long 

 glass tube is attached to the cut end of the stem, still 

 connected \\'ith the roots, by the use of rul)ber tul:)ing, 

 as shown in figure 4s, and a very small f]uanlit\" of water 

 lUaA' lie poured in to moisten the cut end of the stem. 

 In a few minutes the water begins to rise in the glass 

 tube. In some cases it rises cjuite rapidly, so that the 

 coluirm of water can readilv l>e seen to e.xtenfl higher 

 and higher tip in the tube when obser\'ed at C|uite 

 short intervals. (To meastire the force of rof)t pressure 

 is rather difhcult for elementary' work. To measure it 

 see Ganong, Plant Physiology, pp. 67, 68, or some other 

 book for advancefl work.l 



67. In either case where the experiment is 

 continued for several da}'s it is noticed tliat the 

 column of water or of mercury rises and falls at 

 different times during the same dav, tliat is, the 

 column stands at varvinn; heights; or in other 

 words the root presssure varies during the dav. With some plants 

 it has been found that the pressure is greatest at certain times 

 of the day, or at certain seasons of the year. Such variation 

 of root pressure exhibits what is termed a periodicity, and in 

 the case of some plants there is a daily periodicity; while in 

 others there is in addition an annual periodicity. With the 

 grape vine the root pressure is greatest in tire forenoon, and 

 decreases from 12-6 p.m., while with the sunflower it is greatest 

 l)eforc 10 A.M., when it begins to decrease. Temperature of 

 the soil is one of the most important external conditions affect- 

 ing tlie activity of root pressure. 



Fi?. 45- 

 Experiment tf) 

 show ruot pressure 

 (Detmer). 



