The Structure of Leaves. 9 



I need hardly say this Alphabet is " Anatomy." Now the 

 general structure of the leaf is liajipily not couiplicutcd, and can 

 be explained without employing any long botanical terms. Tlie 

 leaf, like every other part of the plant, is conijioscd of two distinct 

 tissues or fabrics, viz., that which forms the jjulp, and that which 

 forms the woody part, z.e., the "veins" of the leaf. The veins 

 give support to the ])ulp, much in the same way as the bones in 

 the human frauie support the flesh, or as the trunk of a tree 

 supports the branches. 



There is, besides, an epidermis or skin spread over both the 

 upper and the under surface of the leaf, which also covers almost 

 without exception* every other part of the plant. This then is all 

 that I need remark on the anatomy of the leaf. Let us now, having 

 completed our alphabet, proceed to the more interesting portion of 

 this study. Let us first trace the growth of the leaf from the very 

 earliest stage in which the microscope can discover it. At its first 

 appearance each leaf is a minute projection of "pulp," conical in 

 shape, covered and protected by two sheaths or scales. As this 

 projection increases in size, the pulp inside the sheath gradually 

 assumes a more leaf-like form, the process of formation beginning 

 from the top of the future leaf downwards. 



Next the veins begin to grow and give a more definite outline 

 to the pulp, which still lies curled up under the protecting sheuth. 

 Finally, the stalk appears, which, by its increase in the spring, lifts 

 up the rest of the leaf above its protecting scales. These scales, in 

 many plants, remaining throughout the summer at the bottom of 

 the leaf stalk, are known to botanists by the name of " Stipules." 



The formation of the leaf begins the year before it is full grown. 

 This is the case, in order that no vital process may be going on in 

 the plant during the winter, lest frost or cold should injure the 

 delicate machinery, and that without delay the leaf, being formed 

 and safely protected, may be ready to spring forth directly winter 

 is over. 



From this it is evident how large a share belong to the veins in 

 defining the shape of the leaf; they are not, however, entirely 

 responsible for it, because the pulp, as was said before, is partially 

 developed before the veins appear at all. It is, however, pretty 

 clear that while the general outline of the leaf is dependent on the 

 pulp, the cuttings on the margin are due to the veins. For these, 

 as they extend away from the centre of the leaf, stretch the pulp so 

 as to make it cover them; if then the pulp is at all scanty, or if it 

 grows but slowly, in proportion to the veins, it is clear that when 

 it is protended in one direction, it will be indented in the opposite 



• Except tlie stigma. 



