[207] 



Mitbcreb Xeavea* 



By J. W. Fisher. 

 Plate 39. 



THE botanical definition of a leaf is " a thin flattened expan- 

 sion of epidermis, containing between its two layers vas- 

 cular and cellular tissue, nerves, and veins, and performing 

 the functions of exhalation and respiration." The elementary 

 organs of which all vegetable structures are composed, are simple 

 cells, placed in juxtaposition or superposition. These cells assume 

 many different forms according to the position they are to occupy, 

 and their contents are almost as varied as their shapes, adapting 

 them for the purpose they are intended to serve in the vegetable 

 economy. We might trace the cell from its most simple form in 

 some of the microscopic fungi, through all its different modifica- 

 tions into the most highly organised vegetable structures, did time 

 permit. Let it suffice, however, to point out that the outer or 

 epidermal coverings of the leaves of trees, with which we are now 

 more particularly concerned, are, under the microscope, found to 

 consist of cells, placed more or less closely together, or in juxta- 

 position, forming a flattened expansion, and containing between 

 them the vascular and cellular substance of the leaf. 



If we compare a leaf to an animal, the epidermis will represent 

 the skin, the ribs and veins branching off from the mid-rib will be the 

 bony skeleton, and the inner substance, with its varied cell contents, 

 will stand for the flesh and internal organs. A closer examination 

 of the epidermis will show the presence of numerous crevices or 

 pores between the cells, formed by two or four guard-cells, which 

 are crescent-shaped, and smaller than the epidermal cells, afford- 

 ing a means of communication between the outer atmosphere and 

 the inner substance of the leaf. These pores or stomata appear 

 to perform the twofold functions of respiration and the transmis- 

 sion of fluids. The cellular mass intervening between the two 

 layers of epidermis consists of loosely packed cells containing 

 chlorophyll, the colouring matter of the leaves, which is visible 



