NUTRITION. 135 



surfaces of some ? What relation does this repulsion of water have to 

 blocking of stomata by rain ? 



Demonstration. — To show the conditiotu affecting evaporation. 



Construct a potometer as follows : Bend the central stem of a J-tube 

 imtil it is parallel with the cross piece. Fit into the lower opening of 

 the straight leg a capillary tube 30-4.0 cm. long, with 3 cm. of each end 

 bent at right angles to the main part and in opposite directions. Into 

 the bent leg fit a shoot of a thrifty plant cut off under water, at the 

 same time filling the J-tube with water. (To accomplish this, bend the 

 shoot to be cut off so that the place of the cut is submerged in a deep 

 pan of water. Fit it in tube without exposing cut siu-face at all to air.) 

 Dip the lower end of the capillary tube in water and allow apparatus 

 to stand until capillary tube fills with water. Remove the water for a 

 moment and allow a bubble i cm. long to enter ; time it as it moves be- 

 tween a series of equidistant marks on capillary tube. Try the rate 

 under various conditions of light, temperature, and moisture acting on 

 shoot. 



To show loss of liquid water when absorption is great and evaporation 

 slow. 



Grow seedlings of wheat or oats until 5-10 cm. high ; then cover with 

 a glass bell for an hour or two. Where do drops of water appear ? 

 Why? 



B. Foods in general. 



179. Foods. — In addition to an adequate supply of water, 

 plants require food. To be a food, the material must consist of 

 certain elements put together in such proportions and in such 

 a way that it can be used, either at once or by the expendi- 

 ture of comparatively little work upon it, to repair or renew 

 the living protoplasm. All foods are compounds of carbon 

 with two to four other elements. The best foods are very 

 complex in their construction. Only the smallest and sim- 

 plest plants can live upon the simpler carbon compounds. For 

 most plants the proper foods are precisely of the same nature 

 as for animals, and though each sort of plant has certain 

 kinds of food which it can use best, it may be fed with many 

 different kinds 



