Loss of Water 



which they depend, are too indefinitely known to 

 warrant any description of them in a simple account 

 such as this, and it must suffice to say that the iron, 

 potash, calcium, and magnesium which enter the 

 plant and are known to be essential to it, are of use 

 in promoting the chemical changes which result 

 finally in the building up of the living stuff of the 

 plant, and in its subsequent changes which are 

 expressed in what we term living activities. 



The second special function of the leaf, the 

 passing-off of water, is necessary in order to con- 

 centrate the various earth-salts sufficiently to make 

 them useful, and to keep the plant cool. 



The solution taken in from the soil is as a rule, 

 and in the vast majority of plants, exceedingly weak. ^ 

 In order to obtain enough earth-salts much greater 

 quantities are therefore taken in than are necessary 

 to distend the cells and to provide water, as one of 

 the raw materials for food-making, and the surplus 

 must be got rid of. 



The burning rays of a vertical sun in a cloudless 

 sky would soon raise the temperature of the plant- 

 body above what the living stuff can withstand 

 but for the fact that much of the heat is used in 

 evaporating water which passes off through the pores 

 in the leaf, which we have already mentioned. 



Water is essential to aU plant activity. The 

 living stuff of the plant can have no active existence 

 unless it is saturated and distended with water ; 



14 



