216 EXPERIMENTS WITH PLANTS 



remove the leaf, weigh, seal the cut surface with vase- 

 line or grafting wax and reweigh; expose the various 

 leaves simultaneously to the same conditions for the 

 same length of time (five or six hours in sunshine), 

 and weigh again; divide the loss in weight (i. e., the 

 difference between the results of the second and third 

 weighings) by the original weight (results of the first 

 weighing) to get the percentage of loss. Compare the 

 percentages for the different leaves (e. g., a leaf of 

 Squash with that of Hen -and -chickens). 



Plants which have adapted themselves to dry situ- 

 ations have, as we have seen, a surprising ability to 

 retain water. In all of the thick -leaved, fleshy plants, 

 the so-called succulents, sufficient water is stored up 

 in reservoirs (consisting of spongy colorless tissue) in 

 the interior of the leaf to last from one season of rain 

 to another, often for a period of months. Such is the 

 case with the Hen -and -chickens. Live-forever, Ice 

 Plant, etc. In many of these, gummy, mucilaginous 

 substances or hygroscopic salts are present which hold 

 the water tenaciously. Try the experiment of exposing 

 on a piece of glass drops of pure water, together with 

 drops of fluid expressed from such leaves or drops of a 

 strong solution of salt, sugar or gum arable. Which 

 is the first to completely evaporate? 



Along with modifications in structure go changes of 

 position (e. g., from horizontal to vertical) which di- 

 minish evaporation. Notice the vertically placed leaves 



