THE LEAF 59 



leaves oi the floral region ^.^hich, however diverse in appearance, are 

 ail none the less leaves. 



The leaf differs habitually from the stem in its sj'mmetry, though 

 this difference does not apply to all cases, and cannot be held as in 

 itself distinctive of the one from the other. The stem is usually radial 

 in symmetry', being developed equally all round. But the leaf is as 

 a rule a flattened organ, showing what is described as dorshentral 

 symmetry, having more or less clearly defined surfaces. In most 

 leaves one face is turned upwards to the sky, and the other down- 

 wards ; but this is by no means constant. It is important for 

 clearness of description to distinguish these two faces by some more 

 constant character than that of direction up or down. The most 

 constant is their relation to the axis which bears the leaf. One of the 

 faces is directed in the young bud towards 

 the axis which bears it ; it is therefore 

 defined as the ad-axial, though as it is 

 usually directed upwards it is often called 

 the upper surface. The other faces aw-ay 

 from the axis in the bud, and is defined as 

 the ab-axial, and as it is usually directed 

 downwards it is styled the loiver. But the ^ . , 



terms "upper" and "lower" are merely 

 descriptive of position : they are not scientific ^^ 



definitions as the others are. The Common Transverse section of the petiole 



^ ,. , 1 J 1 . ... of tile Sunflower. ( x- 6.) 



Garlic shows how necessary this precision is, 



for in most of its leaves, by a twist of the leaf-stalk, the ad axial 



face is turned downwards. 



Since the tissues of the leaf are continuous at the base with 

 those of the stem, the constitution of the petiole may be expected 

 to resemble that of the stem. But its form is more or less 

 flattened or channelled on the upper (ad-axial) surface. This is in 

 accordance with the mi.chanical requirements ; for it has to support 

 the weight of the blade, and hold it with some degree of firm- 

 ness in its horizontal position. The necessary strength is secured 

 by a form which in transverse section would appear semicircular, 

 or semilunar, as it is in such a leaf as the Sunflower (Fig. 42). 

 In this case it is traversed by three large vascular strands, which with 

 some smaller strands are arranged in a semicircle. The surrounding 

 tissues are essentially hke those of the young stem, but there is no 

 stelar tract defined by a general endodermis ; here each strand is 

 surrounded by its own sheath. Naturally the xyiem of each strand 



