104 BOTANY. 



out of openings in the stem when the temperature suddenly rises ; 

 ■when the temperature suddenly falls, as at night, there is a suction 

 of water or air into the stem. When the temperature is nearly uni- 

 form, whether in winter or summer, there is no flow of sap. 



VI. Root-Pressure. — Here maybe noticed what is called "root- 

 pressure," which, though not connected with the air humidity, has 

 some relation to the movement of water in the plant. If the root of 

 a vigorously growing plant be cut off near the surface of the ground 

 and a glass tube attached to its upper end, the water of the root will 

 be forced out, often to a considerable height. Hales, more than a 

 hundred and fifty years ago, observed a pressure upon a mercurial 

 gauge equal to 11 meters (36.5 ft.) of water when attached to the root 

 of a vine (Vitis). Clark (1873), in a similar manner, found the pres- 

 sure from a root of a birch (Betula lutea) to be equal to 25.8 metres 

 (84.7 ft.) of water. This root-pressure appears to be greatest when 

 the evaporation from the leaves is least ; in fact, if the experiment is 

 made while evaporation is very active, there is always for a while a 

 considerable absorption of water by the cut end of the root, due prob- 

 ably, to the fact that the cell-walls had been to a certain extent robbed 

 of their water by the evaporation from above. Eoot-pressure is 

 probably a purely physical phenomenon, due to a kind of endosmotic 

 action taking place in the root-cells. 



Practical Studies. — (a). Collect a quantity of green grass in the 

 middle of the day when it is not wet ; weigh it accurately, then thor- 

 oughly dry it in an oven, being careful not to scorch it. Weigh 

 agaiu : the difference in the two weighings will be approximately 

 the amount of water in the living plant, although some water will 

 still be left in the plant by ordinary drying. 



(&) Weigh a handful of beans ; put them into warm water or 

 moist earth for a day or two until they are beginning to sprout. 

 Then gather them up carefully, wipe off all external dirt and mois- 

 ture, and weigh again. Here the difference will be approximately 

 the amount of water absorbed by the protoplasm, 



(c) Place some specimens of Green Slime or Pond Scum on a dry 

 glass slip, using no cover-glass. Note with the microscope the rapid 

 evaporation of water as shown by the collapsing of the cells. 



(d) Gather fresh leaves of clover ; suspend some of them under a 

 bell-jar or inverted tumbler which stands in a plate containing a little 

 water. Put the other leaves into a dry plate with no protection from 

 the dry air. Note that the evaporation is very much more rapid in 

 the dry air than in the moist air under the bell-jar. 



(e) Strip ofE the epidermis from a leaf (hyacinth, live-for-ever, etc., 

 are good) and note that the evaporation is much greater (as shown 



