132 OUTLINES OF PLANT LIFE. 



on the opposite side. The two must be so placed and at such a depth 

 that all the tissues are severed. Support the branch or stiffen it against 

 breaking by bandaging it with strips of wood. Make same observations 

 as in A. Examine the pith. Is it alive ? Does it contain water ? In 

 what tissues, therefore, do you infer water travels to leaves ? 



To show restoration and maintenance of an interrupted evaporation 

 stream. 



Fit a well wilted shoot into the short arm of an unequal tl-tube filled 

 with water to the level of the short end. Allow it to stand for half an 

 hour. Does the shoot recover ? If not, pour mercury into the longer 

 arm until it stands lO cm. abttve,Jts level in the short arm. Does the 

 shoot now recover turgor ? Why ? Allow it to stand for some days. 

 Does the level of the mercury change ? 



175. Motive power. — The force by which water is raised 

 in the larger plants remains yet to be ascertained. The water 

 does not flow along the ducts in a continuous current, as the 

 blood in the veins, propelled by a force behind, for root 

 pressure is not adequate to push it to the height attained. 

 On the contrary, during the times of most active evaporation 

 from the leaves, i.e., when the greatest supply is needed, 

 root pressure becomes almost or quite negative. Capillarity 

 is also inadequate. The diameter of the largest ducts is too 

 small and the resistance to the flow consequently too great to 

 permit the movement, by this means, of a sufficient amount of 

 water to supply the evaporation. The most recent researches 

 point to the evaporation of water from the leaves as a very 

 important or even the chief factor in lifting the water. That 

 the movement is not the work of living cells is shown by ex 

 periments in which stems of plants have been subjected to 

 poisonous agents, or heatea for many hours to a degree suf- 

 ficient to kill al! the living cells, yet without materially affect 

 ing the suppl) o water to the leaves, 



EXERCISE XXVI. 

 To show tht lifting powet oj evaporation. 



Cut off under water a shoot from a thrifty plant ; fasten it air-tight in 

 the end of a piece of glass tubing 30 cm. long, ol appropriate diameter, 



