LEAVES 



191 



open system of veins may be entire, but often develop a 

 toothed margin in connection with the free endings of the 

 vein system, which means that toothed and lobed leaves 

 (Fig. 154, B) are characteristic of Dicotyledons. Leaves 

 with an open system of veins may not only become toothed 



or lobed, but the lobing may be _ 



carried so far that the blade be- 

 comes discontinuous (Fig. 156), 

 and appears as if broken up into 

 several blades (leaflets). Such 

 leaves are said to be compound, 

 but they are better called branch- 

 ing leaves. The results of this 

 branching habit of leaves are 

 related to the character of the 

 vein-system. In pinnate leaves 

 with open venation, branching 

 may result in elongated forms 

 with very numerous leaflets; 

 while in palmate leaves with open 

 venation, branching results in 

 broad forms and a more restricted 

 number of leaflets. This differ- 

 ence and the reason for it be- 

 come very evident when the 

 pinnately branching leaf of black 

 locust (Fig. 156, A) is compared with the palmately 

 branching leaf of red clover (Fig. 156, B). Of course the 

 branches in each case may branch again, and thus the leaf 

 may become quite extensive, with very numerous leaflets. 

 The most familiar illustrations are probably the extensively 

 branching leaves of many ferns. 



120. Internal structure. The foliage leaf is a very 

 efficient machine, and it is necessary to understand the general 

 arrangement of its cells. To observe this most easily, thin 



FIG. 157. Cross-section of a lily 

 leaf, showing the epidermis above 

 and below, with three stomata in 

 the lower epidermis ; between 

 the epidermal layers the meso- 

 phyll (the cells containing chloro- 

 plasts), with a layer of palisade 

 cells against the upper epidermis, 

 and below this the spongy tissue 



' with air-spaces of various sizes 

 (the stomata open into three 

 large ones) ; imbedded in the 

 mesophyll are sections of two 

 veinlets, the group of xylem ves- 

 sels (water-conducting) being at 

 the upper side of each veinjet. 



