Ch. Ill, 2] STRUCTURE OF LEAVES 17 



thin, flat, and green, and perform the function of food forma- 

 tion, other kinds exhibit different features and other func- 

 tions, as familiar, for instance, in tendrils and pitchers. 

 Likewise there are parts which seem to be leaves but are not, 

 as in case of some flattened stems, and even roots ; for leaves, 

 while the principal, are not the only green parts of plants. 



2. The Structure of Leaves 



Typical, or foliage, leaves, despite their external multi- 

 formity, possess an essentially uniform anatomical struc- 

 ture, as shown by comparative observation. 



The most conspicuous and important part of the leaf, that 

 in which the food is formed, is the green tissue, called chlo- 

 renchyma, which is singularly uniform in thickness, texture, 

 and color throughout the leaf blade. Its distinctive green 

 color is not, however, an integral part of its structure, but 

 a separate and easily removable substance. One has only 

 to place a leaf in a glass dish, cover with alcohol, stand in 

 a warm place, and leave for a time, when the green will 

 come out in a beautiful clear solution, leaving the leaf a 

 uniform white. This soluble green substance is called 

 chlorophyll, and is one of the most important substances 

 in nature, as will presently appear. 



Second in prominence is the system of veins, which 

 ramify everywhere throughout the chlorenchyma. They 

 are essentially bundles of tubes which conduct materials 

 into and out of the chlorenchyma. Most commonly they 

 taper and branch from the base of the blade towards the 

 margin, simultaneously producing small veinlets which 

 interlace to a network, as seen very clearly when held up 

 against the light. In other kinds of leaves, such as Grasses, 

 the main veins are uniform in size, and run parallel, or 

 gently curving from base to tip, the veinlets in this case 

 being minute or even wanting ; and such leaves are called 

 parallel-veined in distinction from the former, or netted- 

 veined kinds (compare Figs. 1 and 2 with 34). If, further, 



