ORGANS AND SYSTEMS OF ORGANS. 161 



When the incisions are not limited to the margin, but extend 

 more deeply, we distinguish : 



1. Lobed leaves {yol. lohatum), wlien the incisions do not 

 extend quite half-way to the midrib and the margins of the lobes are 

 rounded. 



2. Cleft leaves {fol. Jissum)^ when the incisions do not extend 

 quite half-way to the midrib and the lobes are pointed. 



3. Parted leaves {fol. 'partUuni), when the incisions extend 

 more than half-way to the midrib. 



4. Divided leaves {^fol. seetuin), wd)en the incisions extend to 

 the base or to the midrib. 



All these forms, lobed, cleft, etc., are again sejiarated into 

 2)almately and ])innately lobed, cleft, parted, and divided accord- 

 ing to whether the direction of the incisions is towai'd the Jxise of 

 the blade or toward the onidrih. 



In the compound leaf the blade is divided into entirely separate 

 parts ; each part is called a leaflet {foliohmi). It really seems as 

 though the petiole were branched, each leaflet having a small petiole 

 of its own by which it is attached to the common petiole. We 

 may again \\'a.\q palnnately and plnnately compound leaves. Some- 

 times the difference between a divided simple leaf and a compound 

 leaf is not easily recognized. Wlien we tind the individual leaflets 

 jointed or articidated to the common petiole in a way similar to 

 that in which the latter is articulated with the stem, we may be 

 certain that it is a compound leaf. The leaflets may be entire, 

 dentate, serrate, etc. ; or lol)ed, cleft, parted, etc. ; thus we may 

 have twice or thrice pinnately or palmatelv compound leaves. In 

 the former case the leaflets are called j?>i7?7?fp, in the \a\Xqv pinnidcB. 



Venation^ that is, the arrangement and distribution of vascular 

 bundles in the leaf, is intimately associated with the form of the 

 leaf-blade. Most monocotyledons have jy«r«<5/eZ-vewje(/ leaves ; 

 most dicotyledons have netted-veined leaves. This venation may 

 again be divided into pinnately veined send pahnately veined. 



We will now briefly consider the moditications of the leaf men- 

 tioned in the beginning of this section (h). 



1. Cotyledons (embryonic leaves). Of these the monocotyledons 

 have one, dicotyledons two, and gymnosperom few or many. They 

 constitute the first leaf-like strnctures of the embryo, appearint>- 

 almost without exception as entire lobes. With JSTageli and others 

 we may designate them as tballome lobes, since true leaves make 



