74 THE HARMONIES OF NATURE. 



in the black elder, 63,000 ; in Iris germanica each surface has 

 nearly 12,000 stomata in every square inch ; and in Yucca, each 

 surface has 40,000. What wonders does a close inspection thus 

 reveal in the structure of a leaf whose surface appears uniform 

 and unbroken to the naked eye. On that side of the leaf where 

 the stomata and the corresponding lacunae chiefly abound or 

 exclusively exist, the green cellular parenchyma is always of a 

 looser texture, whilst on the opposite side, where the stomata 

 are either less abundant or entirely wanting, it is more compact. 

 Hence the under surface of the leaves is generally of a less 

 intense green than the upper one, where the cells are more 

 closely congregated. 



The stomata are destined to admit air, not water, which by 

 drenching the leaf would entirely interrupt the process of respi- 

 ration. This danger is effectually guarded against by the 

 boundary-cells of the stoma, (d.d,) which, from their swelling or 

 expanding in moist weather by the absorption of humidity, are 

 able entirely to close the opening, so that no rain can penetrate 

 into the air-chambers, and thus this simple hygroscopic property 

 renders here the services rendered by muscular contraction in 

 closing the cavities of the animal body. 



Where shall we find pliability and firmness more beautifully 

 combined than in the structure of a leaf? A slight breath of 

 air sets it in motion, and this circumstance is naturally very 

 favourable to the respiratory process, as the perpetual agita- 

 tion of the foliage brings it into contact with new sheets of 

 air, and thus facilitates the exchange of oxygen and carbonic 

 acid. But as the green cellular tissue of the leaf, in which the 

 functions of respiration are carried on, is easily torn, a strong 

 framework or skeleton was needed to give it the necessary sup- 

 port, and this is amply afforded by its ribs, which, consisting of 

 bundles of strong, tough, and colourless vessels, proceed from a 

 chief middle-rib, and ramifying over the whole surface, support 

 the green-coloured cellular tissue as firmly as the trunk, sub- 

 dividing into numerous branches, supports a vast crown of foliage. 



To increase the mobility of the leaves without detriment to 

 their strength, their mid-rib frequently forms a long stalk before 

 merging into the body of the leaf, which, being thus freely sus- 

 pended upon its slender and flexible support, easily gives way 

 to the slightest disturbance of the air, as a ship at anchor gently 

 rocks to and fro in the heaving and subsiding waters. 



