ARBOR DAY ITS HISTORY AND OBSERVANCE. 49 



serve also as ducts or conduits by which the crude sap is conveyed to 

 the leaves and by which, when it has there been made into plant food, 

 it is carried into all parts of the tree for its nourishment. Protected 

 and upheld by these expanded woody ribs, the body of the leaf consists 

 of a mass of pulpy cells arranged somewhat loosely, so that there are 

 spaces between them through which air can freely pass. Over this 

 mass of cells there is a skin, or epidermis, as it is called, the green 

 surface of the leaf. In this there are multitudes of minute openings, 

 or breathing pores, through which air is admitted and through which 

 also water or watery vapor passes out into the surrounding atmosphere. 

 In the leaf of the white lily there are as many as 60,000 of these open- 

 ings in every square inch of surface and in the apple leaf not fewer 

 than 24,000. These breathing pori s, called stomates, are mostly on the 

 under side of the leaf, except in the case of leaves which float upon 

 the water. There is a beautiful contrivance also in connection with 

 these pores, by which they are closed when the air around is dry and 

 the evaporation of the water from the leaves would be so rapid as to 

 be harn ful to the tree and are opened when the surrounding atmos- 

 phere is mo : st. 



The green color of the leaves is owing to the presence in the cells of 

 minute green grains or granules, called chlorophyll, which means leaf- 

 green, and these granules are indispensable to the carrying on of the 

 important work which takes place in the leaves. They are more numer- 

 ous and also packed more closely together near the upper surface of 

 the leaf than they are near the lower. It is because of this that the 

 upper surface is of a deeper green than the lower. 



Such, then, is the laboratory of the leaf, the place where certain 

 inorganic, lifeless substances, such as water, lime, sulphur, potash, and 

 phosphorous, are transformed and converted into living and organic 

 vegetable matter, and from which this is sent forth to build up every 

 part of the tree from deepest root to topmost sprig. It is in the 

 leaves also that all the food of man and all other animals is prepared, 

 for if any do not feed upon vegetable substances directly but upon 

 flesh, that flesh nevertheless has been made only as vegetable food has 

 been eaten to form it. It is, as the Bible says, " The tree of the field 

 is man's life." 



But let us consider a little further the work of the leaves. The tree 

 is made up almost wholly of oxygen, hydrogen, and carbon. It is easy 

 to see where the oxygen and hydrogen are obtained, for they are the 

 two elements which compose water, and that we have seen, the roots 

 ar absorbing from the ground all the while and sending through the 

 body of the tree into the leaves. But where does the carbon come 

 from? A little examination will show. 



The atmosphere is composed of several gases, mainly of oxygen and 

 nitrogen. Besides these, however, it contains a small portion of car- 

 bonic acid, that is, carbon chemically united with oxygen. The carbonic 

 10578 4 



