162 THE STEM PROPER 



228. Action of Osmose in the Root. The sap within the 

 root is generally denser than the water of the soil, so there 

 is a continuous osmotic flow from the latter to the former, 

 but within the stem the fluid is more nearly of the same 

 density throughout and the conditions for osmosis are not 

 so favorable, though it probably does take place to some 

 extent. A more efficient cause is generally held to be the 

 force exerted by the upward pressure of water absorbed 

 into the roots, and known as 



229. Root Pressure. Cover a calla lily, young corn- 

 stalk, sunflower, or other succulent herb with a cap of 

 oiled paper to prevent transpiration, set the pot containing 

 it in a pan of warm water and keep it at a gentle heat. 

 After a few hours look for water drops on the leaves. 

 Where did this water come from ? How did it get up 

 into the leaves ? 



Now cut off the stem of the plant six or eight centimeters 

 (three or four inches) from the base. Slip over the part 

 remaining in the soil a bit of rubber tubing of about the 

 same diameter as the stem, and tie tightly just below the 

 cut. Pour in a little water to keep the stem moist, and 

 slip in above a short piece of tightly fitting glass tubing. 

 Watch the tube for several days and note the rise of water 

 in it. The same phenomenon may be observed in the 

 " bleeding " of rapidly growing, absorbent young shoots, 

 such as grape, sunflower, gourd, tobacco, etc., if cut off 

 near the ground in spring when the earth is warm and 

 moist. This flow can not be due to transpiration, since 

 the leaves and other transpiring parts have been removed. 

 Transpiration, by causing a deficiency of moisture in cer- 

 tain places may influence the direction and rapidity of the 

 current, but does not furnish the motive power, which 

 evidently comes, in part at least, from the roots, and is 

 the expression of their absorbent activity. 



230. Root Pressure and Root Pull. There is no antag- 

 onism between these two forces. Root pull affects the 

 body of the plant with its system of tubes and cells ; root 



