80 COMPOUND ORGANS OF PLANTS. 



structure. That nature has been less elaborate in their forma- 

 tion, will be evident to any one who will only take the trouble 

 to compare a lily leaf with that of a rose. If held up to the 

 light, the intricate and highly complex ramifications of the 

 fibrous structure of the exogenous leaf of the rose will be seen 

 in striking contrast with the extreme simplicity of the endo- 

 genous leaf of the lily. Two different modes of venation have 

 also been noted in endogens, the curve-veined and the straight- 

 veined. In the first instance, the veins run in parallel curves 

 from the base to the apex of the leaf, and in the other case 

 proceed in right lines. The plantain and Hemerocallis or 

 day-lily, are good examples of the first, and grasses of the last 

 method of venation. The margin of endogenous leaves is inva- 

 riably entire, and never marked with indentations of any kind. 

 3. The fork-veined leaves, which are peculiar to ferns, plants 

 still lower in the scale of organization. It may be proper to 

 qualify these divisions and sub-divisions, by remarking that 

 they are not intended accurately to define the boundaries 

 between the different modes of venation. There is an 

 approach to the forked method of venation in some exo- 

 genous plants, as in clover, and doubtless there are many 

 intermediate forms. All classification is but an approxima- 

 tion to that order which obtains in nature. All that Decan- 

 dolle intended, was to point out some of the principal modes 

 in which the woody matter of leaves was distributed through 

 their parenchyma, and to call attention to the fact that the 

 variety of their form is the result of one or the other of 

 these modes of distribution. The student will now under- 

 stand that leaves assume the linear, lanceolate, ovate or 

 orbicular form, according to the greater or less degree of 

 divergence of the woody fibre constituting their framework. 



