20 



CHAPTEII III. THE LEAF. 



20. If we look at the foliage leaf 



of say the Teak tree we at once recognise that it consists 

 of two principal parts (1) the expanded apical portion called 

 the blade or lamina and (2) the stalk, or petiole. In some 

 Nerves. plants the leaves have no petiole and they are then said 

 to be sessile, whereas leaves with a distinct stalk are petiolate. 

 On looking at the lamina we find a framework of firm ribs 

 traversing its surface in all directions, between which the 

 soft green tissue is stretched, much like the cloth over the 

 ribs of an umbrella. The largest and most prominent of these 

 are usually distinguished as ribs, nerves, or veins, and the 

 smallest as veinlets. For the present we may include them 

 all under the general term strands. These strands of firm 

 tissue, if followed up, will be found to extend from the 

 leaf blade through the petiole and stem down to the root, the 

 strands in the leaf being, in fact, branches of similar stouter 

 strands in the stem, which are the so-called vascular bundles. 

 These give the necessary rigidity and strength to the plant 

 enabling the stem to stand erect and the flat leaf-blades to 

 remain extended in the sunlight. These cords of tissue also 

 contain minute pipe-like structures which will be considered in 

 detail later, but through some of which, it may now be noted, 

 water with mineral salts in solution passes from the roots to 

 the green leaf, while others bring back into the stem the food 

 materials manufactured in the leaf by the help of the green 

 chlorophyll under the influence of sunlight, and carry them 

 to those points where they are required. When leaves are 

 cast oft', the places where the vascular strands pass into the 

 stem from the leaf are usually distinguishable on the resulting 

 leaf -scars as more or less, evident dots, or rounded marks. 

 The number, shape, and arrangement of these marks on the 

 scar are often very characteristic. 



Veration. 21. The way in which the strands 



are arranged in the leaf is important and is called the venation 

 of the leaf. Two main classes of leaves are usually distin- 

 guished, viz : 



(1) Parallel -veined. 



(2) Reticulate- or net-veined. 



In parallel venation all the strands which can be easily seen 



