ORGANS AND SYSTEMS OF ORGANS. 161 



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

 more deeply, we distinguish : 



1. Lobed leaves (fol. lobatum), when the incisions do not 

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

 rounded. 



2. Cleft leaves (fol.fissum), when the incisions do not extend 

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



3. Parted leaves (fol. partitum\ when the incisions extend 

 more than half-way to the midrib. 



4. Divided leaves (fol. sectum), when the incisions extend to 

 the base or to the midrib. 



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

 palmately and pinnately lobed, cleft, parted, and divided accord- 

 ing to whether the direction of the incisions is toward the base of 

 the blade or toward the midrib. 



In the compound leaf the blade is divided into entirely separate 

 parts ; each part is called a leaflet (foliolum}. 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 have palmately and pinnately compound leaves. Some- 

 times the difference between a divided simple leaf and a compound 

 leaf is not easily recognized. When we find the individual leaflets 

 jointed or articulated 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 lobed, cleft, parted, etc. ; thus we may 

 have twice or thrice pinnately or palmately compound leaves. In 

 the former case the leaflets are called pinnce, in the latter pinnidce. 



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 parallel-veined leaves ; 

 most dicotyledons have netted-veined leaves. This venation may 

 again be divided into pinnately veined and palmately veined. 



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

 tioned in the beginning of this section (b). 



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

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

 constitute the first leaf-like structures of the embryo, appearing 

 almost without exception as entire lobes. With NAGELI and others 

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



