PLANT NUTRITION. oY 
a needle. Sometimes it is all in one piece, the ‘‘ 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, which 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 minutie 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 
