38 A TEXTBOOK OF GENERAL BOTANY 
midrib. Their position just exterior to the thin-walled phloém 
affords mechanical protection to the latter. 
Veins. The structure of large veins is similar to that of 
a midrib. The complexity of the structure decreases with the 
size of the veins until, near their ends, small veins consist of 
only one or a few conducting cells (Fig. 27). The cells of the 
chlorenchyma are usually arranged so that the conduction of ma- 
terials to and from the veins is facilitated (Fig. 28). In Fig. 29 

Fig. 31. Section of a leaf of Indian corn (Zea mays), showing the simi- 
larity in structure of the two sides of a vertical leaf. (x 285) 
are shown the arrangement of the netted veins and the free 
ends of the veinlets in a small portion of a dicotyledonous leaf. 
Vertical leaves. The leaves of many species of Kucalyptus, 
instead of spreading out horizontally, hang down vertically. 
Both surfaces of the leaf, therefore, receive direct sunlight, and, 
in keeping with this fact, palisade chlorenchyma is developed 
on both sides, as shown in Fig. 30. 
The erect leaves of grasses are very efficient and interesting 
types of photosynthetic organs. When grasses grow close to- 
gether, the erect position of the leaves allows light to pass 
between them and to illuminate a large amount of surface. 
Moreover, both surfaces usually receive direct sunlight. The 
morphologically upper and lower halves therefore have similar. 
functions, and the structure is likewise similar. In this type of 
