140 BOTANY FOR BEGINNERS CHAP. 



plenty of light. At the end of some twelve weeks pull up the plant, 

 take the piece of limestone out, and wash it. Note 



(i) The markings on the limestone show where the roots have 

 touched. 



(ii) These markings have been produced by the acid sap which the 

 roots gave out. 



(iii) In the soil under ordinary conditions the acid sap performs the 

 same kind of work. 



Conditions Necessary for Absorption. 



i. The air which surrounds the plant must have a certain 

 temperature. There is a minimum temperature below which no 

 absorption will take place, and a maximum above which this 

 process will cease. Between these two extremes a temperature 

 can be found for each plant at which the process is most 

 vigorous, and this temperature is said to be the Optimum 

 temperature, 



2. The soil (or culture splution) must also have a certain 

 temperature before absorption can take place. The roots of a 

 plant take in very little water in winter because the soil is very 

 cold ; in summer a larger quantity is taken in because the soil 

 is warmer. 



3. The strength of a culture solution has a very decided 

 effect on absorption. If the solution is very strong the plant 

 cannot take it in ; absorption only goes on when the solution 

 is very weak the condition found in the soil in ordinary 

 circumstances. 



Plants give out Moisture. Plants not only take in water 

 but they also give it out. This is shown by the atmosphere of 

 forests always being moister than places without vegetation. It 

 has been calculated that a good-sized Oak tree will give out 

 in a single day several gallons of water, and during the active 

 life of a Sunflower plant, it will give out 200 times its dry 

 weight of water. That plants lose water is shown by cutting a 

 branch, weighing it, and placing it in a dry place. A second 

 weighing in the course of a few hours will show it to have lost 

 weight. 



Transpiration. The way in which the plant gives out the 

 moisture must now be considered. This giving out of moisture 

 by a plant in the form of vapour is called transpiration. It 

 is only those parts of plants which are in contact with the air 



