PLANT NUTRITION. 27 



a needle. Sometimes it is all in one piece, the e< blade " 

 being unbroken at the edge, or variously notched and 

 indented ; at other times the blade is made up of few or 

 of an infinite number of separate segments or leaflets. 

 If the blade is in one piece, it is " simple," as the leaf of 

 wheat ; if in many pieces, as the leaf of clover, sainfoin, 

 or tares, it is "compound." Very often it has a stalk 

 or " petiole," sometimes it has none. Sometimes it has 

 appendages at its base called " stipules," well seen in 

 vetches or clover ; while the leaves of all grasses, includ- 

 ing all the cereals, are provided with a little mem- 

 branous tongue or outgrowth from the junction of the 

 sheathing stalk of the leaf with the blade, whicli is called 

 a "ligule," and which, though often overlooked, is of 

 some moment to the grazing farmer, as affording one 

 means 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 bun- 

 dles. 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 con- 

 tinuous tubes. There are many kinds of "vessels," but 

 all of them originate from cells. The fibro- 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 number of pores or openings called "stomata," 

 the number, arrangement, size, and form of which vary 

 very much in different plants ; suffice it here to say that 



