BREATHING-PORES. 103 



Calandrinia are the most common representatives of 

 the order. 



While the principal part of the plants belonging to 

 the Chickweed tribe is found in meadows, or shady 

 spots, or in situations and climates where they are 

 abundantly supplied with moisture, the Purslane tribe, 

 on the contrary, chiefly rejoices in hot dry exposed 

 places, where they will flourish at a time when every 

 thing else has fallen a victim to drought and heat. 

 They owe this power to the peculiar nature of their 

 stems and leaves ; which, as I have already men- 

 tioned, are succulent. They require, in consequence, 

 a peculiar mode of cultivation, which I will now ex- 

 plain to you. 



When I gave you in my tirst letter a brief account 

 of the minute and beautiful arrangements by which 

 leaves are able to perform their vital actions, I omit- 

 ted to say anything about their breathing-pores or 

 STOMATA. The time has now come when I must tell 

 you w^hat they are. 



All leaves are covered by an exceedingly thin and 

 delicate skin, which you may often peel off". If you 

 put a small piece of this in water, and look at it w ith 

 a microscope against the light, you will remark a 

 number of very small roundish or oval spaces, through 

 the middle of which a line passes. And if you have 

 patience enough to look at a great many of them, 

 you will in time perceive some of them opening a sort 

 of mouth at the place where the line is. These are 

 what are called breathing-pores ; they are organs by 

 which the leaves inhale and exhale air, or vapour 



