86 



^•■■V 



THE LEAF. 



are always connected by simple transverse veinlets. Ex. grass 

 lily. . 



1 2 3 4 



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



o(\. : Forked-veined, as in the Ci'jqitogamia, wlien the veins 

 divide and subdivide by forked divisions which do not unite 

 again. Ex. ferns." 



230. Of the first kind of venation, the reticulttle, tlicre arc two varieties wliicli 

 desenc the most careful attention, /rhc feaUw-veined and the radiate-vcina'. 



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

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

 Ijrn.nching veinulets. Ex. beech, chestnut. "; 



2. In the radiatc-vcintd, 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 veinulets. 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 tlie 

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



§4. FORM OR FIGURE. 



231. That infinite variety of beautiful and graceful foims for ^^'liicli tlic leaf is 

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

 with its venation. Since it is throu^ tjic veins alone that nutriment is con^-eyed 

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

 be tlie gi-eatest extension of outline -where the veins are largest and most numei"- 

 ous. ) Consequently, the form of the leaf \vill depend upon the direction of the 

 veins, and the vigor of their action, in developing the intervening ti'isue.) For 

 this interesting tlicory we are indebted to Alphonse Dc CandoUe. 



