70 



BOTANY OF THE LIVING PLANT 



leaves of the floral region which, however diverse in appearance, are 

 all none the less leaves (see Figs. 260-264, pp. 349-35 1 )- 



The leaf differs habitually from the stem in its symmetry, 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 dorsiventral 

 symmetry, and 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 away 

 from the axis in the bud, and is defined as 

 the ab-axial, and as it is usually directed 

 downwards it is styled the lower. But the 

 terms " upper " and " lower " are merely 

 descriptive of the positions that are usual : 

 they are not scientific definitions as the 

 others are. The Common Garlic shows how 

 necessary this precision is, for in most of its leaves, by a twist of 

 the leaf-stalk, the adaxial 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 mechanical requirements ; for it has to support 

 the weight of the blade, and hold it with some degree of firmness 

 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. 45). 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 like 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. The xylem of each strand 



Fig. 45. 



Transverse section of the petiole 

 of the Sunflower. ( x 6.) 



