26 THE BOOK OF NATURE STUDY 



leguminous plants has been the subject of much investigation in 

 Germany, in the United States, and in England. The discovery 

 by Hellriegel in 1886 that the power of using the free nitrogen 

 depended upon the formation of nodules upon the roots has led 

 to considerable research work. Pure cultures of the organism 

 producing the tubercles on the roots have been obtained, and 

 patient investigation has led to the working out of improved 

 methods of making the cultures. 



ROOT-PRESSURE. The passage of substances in solution in 

 the soil into the cells of the root-hair has already been described. 

 Once absorbed by the root-hairs, the contents pass from cell to 

 cell of the outer layers of the root until the nbro-vascular tissue is 

 reached. These outer cells of the root may be said to pump the 

 water with the salts dissolved in it into the wood, thence it circu- 

 lates through the plant. This is what is meant by root-pressure. 

 That roots do pump water into the wood may be demonstrated in 

 the following way : Take a small fuchsia or geranium plant in 

 a pot, cut the stem across about 3 inches above the level of the 

 soil, and below any of the lateral branches. Attach to the cut end 

 of the stem by means of indiarubber tubing an empty glass tube 

 and water will be forced into the tube to a height of 2 or 3 feet, if 

 root-pressure is very considerable, as it is in the spring. 



In order to measure exactly the force of root-pressure, 

 apparatus such as that drawn in Fig. 12 is used. A T-shaped 

 glass tube is attached by one end of the cross piece to the plant ; 

 by means of indiarubber tubing, the other end is tightly corked. 

 To the centre piece of the tube, a U tube containing mercury is 

 fitted. When the apparatus is set up, the mercury is at the same 

 level in the two limbs of the tube. But as the sap rises from the 

 stem into the T-shaped tube it flows on into the U tube, forcing 

 the mercury into the right-hand limb of the tube. The difference 

 in the column of mercury in the two limbs gives the column of 

 mercury which is supported by the sap, and is a measure of the 

 force of the root-pressure. Regular observation and accurate 

 measurement shows that the root-pressure varies not only at 

 different seasons, but at different times of the same day. 



Root-pressure it is that makes plants bleed when pruned. 



