86 THE LEAF. 



are always connected by simple transverse veinlets. Ex. grasi. 

 lily. 



1 234 



FIG. 28 Forms of venation. 1, 2, Exogens ; 3, Endogen ; 4, acrogen. 



3d. Forked-veined, as in the Cryptogamia, when the veins 

 divide and subdivide by forked divisions which do not unite 

 again. Ex. ferns. 



230. Of the first kind of venation, the reticulate, there are two varieties which 

 deserve the most careful attention. The feather-veined and the radiate-veined. 



1. The feather-veined leaf is that in which the venation con- 

 sists of a midvein, giving off at intervals lateral veinlets and 

 branching veinulets. Ex. beech, chestnut. 



2. In the radiate-veintd, the venation consists of several 

 veins ( $ 227) of nearly equal size, radiating from the base 

 towards the circumference, each with its own system of veinlets 

 and veimilets. Ex. maple, crow-foot. 



3. In parallel venation, the veins are either straight, as in the 

 linear leaf of the grasses, curved, as in the oval leaves of the 

 Orchis, or transverse, as in the Canna, Calla, &c. 



H- FORM OR FIGURE. 



231. That infinite variety of beautiful and graceful forms for which the leaf is 

 distinguished, becomes intelligible to the student only when viewed in connection 

 with its venation. Since it is through the veins alone that nutriment is conveyed 

 for the development and extension of the parenchyma, it follows that there will 

 be the greatest extension of outline where the veins are largest and most numer- 

 ous. Consequently, the form of the leaf will depend upon the direction of the 

 veins, and the vigor of their action, in developing the intervening tissue. For 

 this interesting theory we are indebted to Alphonse De Candolle. 



