LEAVES 



3362 



LEAVES 



a substance called chlorophyll, or leaf green, 

 found in certain of the inner cells (see CHLO- 

 ROPHYLL). Running through the leaf are 

 threads of fiber, forming ribs and veins. In 

 the epidermis are numerous minute openings 

 (stomata), known in popular language as 

 breathing pores, because they influence the in- 

 terchange of air between the outer and inner 

 portions of the leaf. The chief parts of a per- 

 fect leaf are the expanded portion, or the 

 blade, the leafstalk, called the petiole, and two 

 expansions at the base of the petiole, known as 

 stipules. 



Forms of Leaves. Of all the countless leaves 

 borne by plants year after year, no two are 

 exactly alike, but botanists classify them ac- 

 cording to certain prevailing types of forma- 

 tion. Every leaf may be placed in one of two 



apex 



veins 



blades 



petiole 

 stipule 



base 



stipule 



PARTS OF A LEAF 



great divisions, simple and compound, but 

 there are many varieties of both simple and 

 compound leaves. To the former class belong 

 those whose blades form one continuous piece; 

 to the latter, those leaves which consist of 

 several leaflets, each united to a common stalk, 

 or midrib, by its own stem. Simple leaves 

 are sometimes deeply cut into lobes, as in the 

 case of the oak and maple. Compound leaves 

 are said to be pinnately divided when the leaf- 

 lets are arranged along the sides of the midrib, 

 and palmately divided when they radiate from 

 the petiole. The terms pinnate and palmate, 

 as indicated by the illustration on page 3361, 

 refer to the shape of the leaf.; the one is de- 

 rived from the Latin pinnatus, meaning feath- 

 ered; the other, from palmatus, meaning 

 marked with the palm oj the hand. 



Ruskin, quoted above, has described, in his 

 picturesque fashion, the wonderful variety of 

 form assumed by the foliage of the plant king- 

 dom. 



Star-shaped, heart-shaped, spear-shaped, ar- 

 row-shaped, fretted, fringed, cleft, furrowed, ser- 

 rated, sinuated, in whorls, in tufts, in spires, in 

 wreaths, endlessly expressive, deceptive, fantas- 

 tic, never the same from foodstalk to blossom, 

 they seem perpetually to tempt our watchful- 

 ness and take delight in outstripping our wonder. 



Some of the most familiar forms of the leaf 

 blades and their marginal variations are shown 

 in the picture on page 3361, and leaves also 

 vary greatly in size, ranging from the minute 

 forms of such plants as the arbor vitae to the 

 giant leaves of many water plants and palms. 

 The leaves of the Victoria regia, a plant found 

 on the lakes of Guiana and Brazil, are six feet 

 in diameter, large enough to provide a stand- 

 ing-place for waterfowl while watching for their 

 prey. A Central American palm of the arum 

 family has been known to bear leaves thirteen 

 feet long. 



According to the character of their veining, 

 leaves are said to be netted-veined and paral- 

 lel-veined. When all the veins of a netted- 

 veined leaf rise from a single rib the leaf is 

 pinnately-veined, and when they run through 

 the blade like the toes of a web-footed bird, 

 the leaf is palmately-veined. Parallel-veined 

 leaves, also, are of two types, those in which 

 the threads run lengthwise through the blade, 

 and those with parallel veins extending from 

 the midrib to the margin (see illustration). 

 The two general types of leaf arrangement on 

 the stem, opposite and alternate, are also shown 

 in the illustration. 



The Work of the Leaf. The most important 

 function of the leaf is the manufacture of 

 sugar and starch for the nourishment of the 

 plant. This is a process which, in some mar- 

 velous way not fully understood, takes place 

 through the breaking up of water and carbon 

 dioxide into their elements. Carbon dioxide 

 is absorbed from the air through the openings 

 in the epidermis; inside the leaf cells is also 

 found a supply of raw material, consisting 

 both of carbon dioxide and water. Energy 

 from the sun's rays is absorbed by the chlo- 

 rophyll (green coloring matter) in the leaf, 

 and this energy is the power which breaks up 

 the raw material into carbon, hydrogen and 

 oxygen. These gases are then united into new 

 compounds, which, after certain chemical 

 changes, become starch and sugar. During the 

 process some of the oxygen passes into the 

 atmosphere. This marvel of plant life can 

 take place only in the sunlight and is charac- 

 teristic only of plants that contain chlorophyll. 

 It is a function not possessed by plants that 



