70 NATURE TEACHING 



Leaves often act as the climbing organs of the plant, 

 and all gradations can readily be found between an 

 ordinary green leaf which has the power to hold on to, 

 or even twist round supports and the special structures 

 of other plants, often so much altered to make them 

 more suitable for this particular use that they have 

 almost lost their leafy character. Thus in the wild 

 clematis (traveller's-joy or old man's beard) so common 

 on chalk and limestone districts, the leaf-stalk twists 

 round objects and holds the plant up ; similarly in the 

 garden nasturtium the leaf-stalks of the ordinary green 

 leaves do the same. In the garden pea only a special part 

 of the leaf the long thin end is of use as a climbing 

 organ, and similarly in many of the vetches. Examples 

 like these are of special interest, as they show us how 

 adaptable the parts of plants are, and how the same part 

 can serve very different purposes. 



Structure of Leaves. 



In most leaves the blade has running through it a 

 number of veins, often conspicuous, especially on the 

 lower side, as ridges. The leaves of the maple, black- 

 berry, and indeed almost any ordinary thin leaves, show 

 them very plainly, and, on holding such leaves to the 

 light, it is seen that there is a perfect network of these 

 veins, the small veins being branches of the larger ones. 

 These veins are really the continuations of the woody 

 tissue which we have already seen in the stem, and are 

 of use as a supporting framework to the soft tissue of 

 the leaf, spreading it out to the light and air, and pre- 

 venting the leaf from being readily torn. They are also 



