THE STEM AND BRANCHES. 187 



nised under the microscope by means of the 

 curious guard-cells which look like lips, and 

 which give the pores, in fact, their strange 

 mouth-like aspect. 



What is the use of these lips? Well, they 

 are employed for opening and closing the evapo- 

 rating pores, or stomata. In dry weather it is not 

 desirable that the pores should be open, for then 

 evaporation should be limited as far as possible. 

 So, under these conditions, the lips contract, and 

 the pore closes. Excessive evaporation at such 

 times would, of course, damage or destroy the 

 Toliage ; the plant desires rather to store up and 

 retain its stock of moisture. But after rain, and 

 in damp weather, the roots suck up abundant 

 water ; and then it becomes desirable that 

 evaporation should go on, and the leaves and 

 growing shoots should be supplied with liquid 

 food, as well as with the nitrogenous matter and 

 salts dissolved in it. Hence at such times the 

 pores open wide, and allow the water in the 

 form of vapour to exude from them freely. 



The object of this evaporation, again, is two- 

 fold. In the first place, it supplements root- 

 pressure as a means of raising water to the 

 leaves and growing shoots ; and in the second 

 place, by getting rid of superfluous liquid, it 

 leaves the nitrogenous material and the food- 

 salts in a more concentrated form, at the very 

 points where they are just then needed for the 

 formation of fresh living protoplasm and other 

 useful constructive factors of plant-life. But how 

 does evaporation raise water from the ground ? 

 Jn this way. The living contents of each cell 



