60 



LEAVES. 



veins always proceed directly from their origin to the margin 

 of the leaf without forming any meshes or net work, as is 

 exhibited by a leaflet of the Aspidium Acrurtichoides (fig. 

 35.) This variety of venation belongs to the ferns, and is a 

 distinguishing characteristic of this class of 

 plants. 



2. Parallel vrincd leaves are those in which 

 the veins proceed from their origin to their ter- 

 mination without any subdivision ; the veins be- 

 ing connected by minute, parallel, straight vein- 

 lets, passing perpendicularly from one to the 

 other. The veins of this variety either run 

 from the base of the leaf to the apex, as in Corn, 

 Lily, Grains, and as seen in fig. 33, a ; or from 

 the midrib to the margin, as in the Carina and 

 Arum Walteri, &c. (fig. 36, b.) Of this latter variety there 



are comparatively few specimens in 

 temperate climates, becoming more 

 abundant as we approach the equato- 

 rial regions. Parallel veined leaves 

 are characteristic of Endogens. 



3. Reticulated or netted veined leaves 

 are those when veins branch and rami- 

 ty^in all directions, and by anastimo- 

 sing with each other form a complete 

 network. This variety of leaves is 

 characteristic of Exogenous plants, 

 and they are the most varied in their 

 forms, and comprise the greatest pro- 

 portion of leaves of temperate climates. 



From the above remarks we observe that the three great 

 classes of vegetables are characterised by distinct modes of 

 venation. Fiovverless plants, or Acrogens, so far as they fall 

 within this subject, are characterised by the forked veined 

 leaves. Endogens are, with {ew exceptions, as in the Trilli- 

 um and Smilax, distinguished by the parallel, and Exogens 

 by the reticulated veined leaves. These are important dis- 

 tinctions for the student to bear in mind, as they may be of 

 much importance to him in many cases, when all other char- 

 acteristics of these divisions may be absent, or obscure at the 

 time of examination. 



67. We now proceed to point out the diflferent varieties of 

 simple leaves, occasioned by the course of the veins. There 

 are two varieties of venation in the reticulated leaves, occa- 



