NUTRITION. 131 



A more satisfactory record may be reached by attaching to the stump 

 a T-tube as shown in fig. 113. To the horizontal arm attach a mercury 

 pressure gage. (A pressure gage may be readily constructed by bending 

 a glass tube, about 5 mm. diameter (3 mm. bore) and 80 cm. long, upon 

 itself 30 cm. from one end, so that it forms a IJ with unequal legs 3-4 cm. 

 apart. Bend 5 cm. of the end of the short leg at right angles, in the 

 plane of the (J- Tie the legs to a piece of cork between the legs near top, 

 so that the tube will not be easily broken by the leverage of the legs on 

 the bottom bend. ) Fill the space between stump and mercury with water. 

 In the third arm insert a short tube drawn out to a slender point to per- 

 mit the escape of air and extra water. Seal this with a flame after filling. 

 There must be at least 15 cm. of mercury in U-portion of manometer. 

 At beginning mark, with a bit of gummed paper, height of mercury in 

 each leg ; measure difference at intervals thereafter until mercury begins 

 to fall. 



174. Route to the leaves. — After entering and traversing 

 the wood strands of the roots, the water is thence trans- 

 ferred along the stem in the same tissues, which are con- 

 tinuous with those of the root. Since the wood strands form 

 an unbroken line to the most remote parts of the leaves, 

 passing out in the ribs and forming the finer veins, the water 

 may be distributed to every part of the plant body. 



Within the wood it travels chiefly in the cavities of the large ducts or 

 vessels, when these are present, though the walls, also, are saturated 

 with it, and permit a slower movement. These ducts, although of great 

 relative length (some up to I m. ), are not continuous tubes like the veins 

 of an animal, nor are they always filled with water. The water is often 

 broken into short columns by numerous gas-bubbles, and in ascending to 

 any considerable height must traverse many cell-walls. 



EXERCISE XXV. 



To show roughly the path of evaporation stream in woody plants. 



A. From a leafy shoot of a woody plant remove a ring of bark 5 mm. 

 wide. With grafting wax protect the exposed surface against drying. 

 Observe whether the leaves wilt or not, and if they wilt, the time re- 

 quired. 



B. With a knife or fine saw cut a little over half through the stem of a 

 plant of the same sort used in ^ ; i cm. above this cut make a similar one 



