40 PHYSIOLOGY. 



which is instrurtive in this respect we shall find in a comparison 

 between the transpiration of water from the leaves of a cut shoot, 

 allowed to lie unprotected in a dry room, and a similar cut shoot 

 the leaves of which have been killed. 



77. AliiTost any plant will answer for tlie experiment. For this purpose I 

 have used the following method. Small branches of the locust (Robinia 

 pseudacacia), of sweet clover (Melilotus alba), and of a helicjpsis were 

 selected. One set of the shoots was immerseil fur a moment in hot water near 

 the boiling point to kill them. The other set was immersed for the same 

 length of time in cold water, sri that the surfaces of the leaves might be well 

 wetted, anil thus the two sets of leaves at the beginning of the experiment 

 would be similar, so far as the amount of water on their surfaces is con- 

 cerned. All the shoots were then spread out on a table in a dry room, the 

 leaves of the killed shoots being separated where they are inclined to cling 

 together. In a short while all the water has evaporated from the surface of 

 the living leaves, while the leaves of the dead shoots are still wet on the sur- 

 face. In six hours the leaves of the dead shoots from which the surface 

 water had now evaporated were beginning to dry up, while the leaves of the 

 living plants were only becoming flaccid. In twenty -four hours the leaves 

 of the dead shoots were crisp and brittle, while those of the living shoots were 

 only wilted. In twenty -four hours more the leaves of the sweet cl<.)ver and 

 of the heliopsis were still soft and flexible, showing that they still contained 

 more water than the killed shoots which had been crisp for more than a 

 day. 



78. It must be then that during what is termed transpiration the living 

 plant is capable of holding back the water to some extent, which in a dead 

 plant would escape more rapidly by evaporation. It is also known that a 

 body of water with a surface equal to that of a given leaf surface of a plant 

 loses more water by evaporation during the same length of time than the 

 plant loses by transpiration. 



79. Structure of a leaf. — We are now led to incjuire why it is 

 that a living leaf loses water less rapidly than dead ones, and 

 why less water escapes from a given leaf surface than from an 

 equal surface of water. To understand this it will be necessary 

 to examine the minute structure of a leaf For this purpose we 

 may select the leaf of an ivy, though many other leaves will 

 answer equally well. From a portion of the leaf we should make 

 very thin cross sections with a razor or other sharp instrument. 

 These sections should be perpendicular to the surface of the leaf 



