NUTRITION. THE MACHINERY. 23 



of distinguishing useful from useless grasses. Further 

 than this we need not go at present in speaking of the form 

 and general appearance of leaves. Nor need we enter very 

 deeply into the minutiae of their structure. All ordinary 

 leaves are flat plates of cells of various shapes variously 

 arranged, and traversed by fibrous bundles. These bundles 

 consist of long tapering cells or fibres filled with woody or 

 other matter, and of rows of similar cells placed end to end 

 in rows, the partitions between the cells being removed, so 

 that they form continuous tubes. There are many kinds 

 of " vessels," but all of them originate from cells. The 

 fibre-vascular bundles, with their wood-cells, bast-cells, and 

 vessels, constitute what are commonly termed the veins of 

 the leaf. Covering over this mass of cells and vessels is a 

 skin or epidermis, consisting of flattened cells usually placed 

 in accurate contact on the upper surface of the leaf, but 

 below, so modified in shape and position as to leave a num- 

 ber of pores or openings called " stomata," the number, 

 arrangement, size, and form of which vary ve*y much in 

 different plants; suffice it here to say that they are in 

 general very numerous. So far, then, there is little differ- 

 ence to be noted between the structural elements of a leaf 

 and those of a root. The root is more or less cylindric, the 

 leaf is more or less flat; but the essential structures, though 

 differently arranged, are pretty much the same, with one 

 or two notable exceptions. The root has no breathing pores 

 or stomata, and the contents of its constituent cells are so 

 far different from those of the leaves that they contain no 

 green colouring matter. 



Chlorophyll. The main and specially important cha- 

 racteristic of the leaf (and of all the green parts of plants), 



