ORGANS OF NTJTEITION. 151 



compound organs of the plant -which possess a definite shape, 

 as the parts of the calyx, corolla, &c. It is absolutely neces- 

 sary therefore that the student should become thoroughly ac- 

 quainted at once with the more important modifications to which 

 the lamina of a leaf is subject. 



According to DeCandolle, the shape of the lamina of leaves 

 depends upon the distribution and length of the veins, and the 

 quantity of parenchyma which is developed between them ; the 

 general outline or figure being determined by the former, and 

 the condition of the margins by the latter. These views have 

 not, however, been confirmed by De Mercklin, who found, in 

 studying the development of leaves, that the veins were not 

 developed till a period subsequent to that of the parenchyma, 

 and that, moreover, the outline was generally established previous 

 to their formation. The outline or figure of the leaf cannot 

 therefore depend upon the veins. While the views of DeCan- 

 dolle may be thus shown to be incorrect in a scientific point 

 of view, still if this be borne in mind, it is convenient, to say 

 the least, to study the almost infinite modifications of the lamina 

 of leaves with reference to his views, as it is always found that 

 there is a mutual adaptation between the venation of the leaf 

 and its general outline.* We shall therefore describe the 

 various modifications of the lamina to some extent after this 

 manner, and in doing so we shall divide our subject into five 

 heads as follows: — 1. Margin; 2. Incision; 3. Apex; 4. Gejieral 

 Outline; 5. Form. 



1. Margin. — We have already stated that the condition of the 

 margin is dependent upon the extent to which the parenchyma is 



Fig. 295. 



Fig. 295. Diagram of the margins of the leaves, a. Bicrenate. 6. Biser- 

 rate. c. Duplicato-dentate. 



developed between the veins of the lamina. Thus if the paren- 

 chyma completely fills up the interstices between them, so that 

 V the margin is perfectly even, or free from every kind of incision, 

 the leaf is entire {figs. 290 and 294), as in the Orchis tribe. 

 When the parenchjTna does not reach the margin, but terminates 



* In the March number for the present year of the Journal of the Royal 

 Microscopical Society, is a paper by Mr. G-orham 'On the Composite 

 Structure of Simple Leaves,' in which the author has fully and with much 

 ingenuity described the results of his researches on leaf structure, which 

 are entirely at variance with those generally entertained by botanists. We 

 must refer those of our readers who are' interested in this subject to 

 Mr. Gorham's paper, as at present it is scarcely one which can be treated 

 of in a students' manual. 



