BOTANICAL: THE UPPER CLASSES. 151 



characteristic of the Creator's wisdom are the air-cells in 

 a plant; and those in the leaf before us, you will see, 

 are countless. One harbinger of summer is the lilac. 

 How plentiful is its foliage! how sweet the well-known 

 blossom it produces ! and what early associations of youth 

 are brought to mind year by year, if only in our father's 

 garden there happened to have been a lilac tree ! 



In one square inch of a lilac leaf there are no fewer 

 than 160,000 of what are called stomata (from the Greek, 

 meaning " a mouth "). They are formed by two half-moon 

 cells, the cusps of which unite around an oval orifice, 

 which opens and shuts exactly as do the millions of air- 

 cells in our lungs; and these, observe, are spread over 

 the entire epidermis, that is, the external cellular cover- 

 ing. In the skeleton of one of these three box-leaves, 

 mounted, not to show 

 stomata, but the mid- 

 rib, and what may be 

 called branches of the 

 leaf, we see a true repre- 

 sentation of our own 

 skeleton. That midrib 

 is an analogy of our 

 great backbone the 

 vertebral column tho 

 main trunk of the ner- 

 vous system, which con- 

 veys the messages we 

 send to the most distant 

 of our bodily members 

 from the great central 



telegraph-station, the 



r Skeleton of black poplar leaf. 



brain. Let us employ 



our highest magnifying power, called a sixth, which 



