202 TRANSPIRATION. 



265. The Use of Solutions of Salts of metals that give a 

 characteristic colour, or bright line spectrum, when placed in a 

 spirit or Bunsen flame and examined with a spectroscope, is a much 

 better method. Make up a litre of 2 per cent, solution of lithium 

 nitrate, and use some of this instead of the coloured water in the 

 preceding experiment ; cut off the uppermost portion of the paper 

 that becomes wetted, hold it in a flame, and with the spectroscope 

 note the characteristic red lithium line. This solution may be 

 used for measuring the rate of the transpiration current. It' pot 

 plants are used, set the plant in good light and for two days do not 

 water the soil ; then saturate the soil with the solution, set the 

 plant in light, and at intervals of about fifteen minutes pick off a 

 leaf, or cut off a small piece of leaf (rinsing the scissors in clean 

 water or under a tap and drying them each time they are used), 

 hold it in the flame, and note the red lithium line with the spectro- 

 scope. Maize plants raised in culture solution answer well ; trans- 

 fer them to the lithium nitrate solution at the beginning of the 

 experiment. By this method it has been found that the height 

 reached by the solution in an hour may be as much as 40 cm. in 

 Maize, 60 cm. in Sunflower, 80 cm. in rooted Willow cuttings, 

 120 cm. in Tobacco. 



266. Does Water ascend in the Walls or in the 

 Cavities of the Xylem Vessels? To answer this 

 question, we may either (1) block the vessels of a stem 

 with wax or gelatine, or (2) strongly compress the stem in 

 a vice so as to narrow considerably the lumina of the 

 vessels. 



Uproot two well-grown Bean seedlings, or other plants, 

 and let them get somewhat wilted. Have ready three 

 dishes containing (a) a mixture of lampblack and warmed 

 wax or cocoa-butter melting at 33 to 35 C. a temper- 

 ature which will not injure the plant ; (fc) water warmed 

 to the same temperature ; (c) cold water. Place one 

 seedling (A) in the warm water, the other (B) in the 

 melted wax ; in each case cut across the stem under the 

 liquid, and note that in B the blackened wax enters the 

 vessels owing to negative pressure (see 269), then trans- 

 fer both plants to the cold water. This will harden the 

 wax in B ; with a razor pare a thin layer from the cut end 

 of the stem, to get rid of the surplus wax and ensure that 

 the walls as well as the blocked lumina are exposed to the 

 water. After some time note that B becomes more wilted 

 than before, while A recovers from its wilted condition. 



