6/6 The Jouynal of Forestry. 



The epidermis is a further check on evaporation. It is composed 

 of thick-walled cells, commonly in one layer, but sometimes even in 

 three (as in the oleander). 



Some exhalation of water-vapour from the leaves is, however, essen- 

 tial, as the only method by which the excessively diluted food absorbed 

 by the roots can be concentrated. This is permitted and regulated by 

 a vast number of curious " breathing-pores " or " stomata." Each of 

 these usually consists of two cells, curved towards each other like the 

 two sides of the letter 0, which are modified epidermal cells, and 

 form an opening communicating with the intercelhdar spaces. In . 

 moist air these cells swell out and enlarge the aperture, whilst as they 

 become drier they shorten and straighten, so as to close it. But the 

 surrounding cells, differing in form, have a contrary influence, so that 

 the result is that of two opposing forces. Stomata occur chiefly on 

 the lower surfaces of leaves away from direct sunshine ; thus leaves 

 soon wither when artificially reversed in position. Stomata do not 

 occur on leaf-surfaces in contact with water, where evaporation is 

 impossible. They vary in number from a few dozen up to about 

 170,000 to a square inch. An apple leaf has about 100,000, one of 

 lilac 700,000, whilst the larger lime has over a million. 



To enable it to shed water readily, the surface of leaves is com- 

 monly protected by a thin natural varnish of wax, or by a powdery 

 *' bloom " of this substance, as in the cabbage, on which water runs 

 like mercury. 



When leaves remain on the tree through the cold season, and until 

 after the appearance of new leaves, so that the stem is never leafless, 

 they are termed " evergreen." In these cases, however, as in deciduous 

 trees, they often last only a year, withering or falling soon after the 

 expansion of the new ones. In conifers they remain on the tree from 

 two to eight, ten, or even twelve years. 



Some leaves wither, but do not fall, whilst others fall un withered. 

 The fall of the leaf is owing to the formation of an articulation, or 

 joint, as under the influence of a surgeon's ligature. The death of the 

 leaf, on the other hand, is due to the choking of its cells and the 

 vessels of the petiole by the mineral matter brought in solution in 

 the w^ater absorbed by the roots, just as in very old human beings the 

 aorta, or great left artery of the heart, becomes choked with bony 

 matter. Chemical modification of the chlorophyll causes the varied 

 hues of autumn. This mineral matter, which renders leaves useless 

 physiologically, also renders them valuable as manure. The dried 

 leaves of the elm contain more than 11 per cent, of mineral matter, 

 while the wood contains less than 2 ; those of willow more than 

 8 per cent., against 0-45 in the wood ; those of beech 6, against 0-36 ; 

 those of oak, 4 against 0*2 ; and those of pitch pine, 315 against 0'25. 



