FOLIAGE LEAVES: FUNCTION, STRUCTURE, ETC. 35 



B. Structure of foliage leaves. 



28. Gross structure. — It is evident that the essential part 

 of a foliage leaf is its expanded portion or Made. Often the 



leaf is all blade (see Figs. 7, 

 8, 18) ; frequently there is a 

 longer or shorter leaf-stalk 

 {petiole) which helps to put 



Fig. 25. Two types of leaf venation. The figure to the left is a leaf of Solomon's 

 seal (Polygonatum), and shows the principal veins parallel, the very minute cross 

 veinlets being invisible to the naked eye, being a monocotyl type. The figure to 

 the right, is a leaf of a willow, and 6hows netted veins, the main central vein (mid- 

 rib) sending out a series of parallel branches, which are connected with one another 

 by a network of veinlets, being a dicotyl type.— After Ettingshausen. 



the blade into better light-relation (see Figs. 1, 9, 17, 20, 

 2G); and sometimes there are little leaf -like appendages (stip- 

 ules) on the petiole where it joins the stem, whose func- 

 tion is not always clear. Upon examining the blade it 

 is seen to consist of a green substance through which a 



