PROBLEM 1 What Part Do Leaves Play in Making 



and Using Foods? 



An interesting experiment. Starch, sugar, 

 protein, and fat are all found in plant 

 protoplasm. You learned this in the last 

 unit. Do they get there from the soil? 

 Are thev made in the plant from soil 

 materials? Just where do they come 

 from? This question of how these com- 

 pounds get into a plant and how a plant 

 grows has interested people for a long 

 time. Early in the 17th century Jan van 

 Helmont, a Flemish physician, performed 

 a simple experiment which helped a little 

 toward the answer. He weighed the soil 

 in a large tub and planted a small willow 

 branch in it. For five years he watched 

 it carefully and watered it regularly with 

 rain water. At the end of this time the 

 branch had grown into a small tree 

 weighins^ more than 160 pounds. Then 

 he weighed the soil once more. He was 

 amazed to discover that the soil weighed 

 only two ounces less than when he 

 started the experiment! The experiment 

 was convincing proof that the soil was 

 not the source of the bulk of the mate- 

 rials used in the growth of the willow 

 tree. Evidently the soil supplied only the 

 tiniest part of the materials used by the 

 willow in its growth. To discover where 

 the rest came from we must study the 

 plant. Let us begin with the leaves. 



Differences in leaves. Some kinds of 

 plants, like the Spanish moss and some 

 kinds of cactuses, have little or nothing 



by way of leaves. The cone-bearing trees, 

 such as the pines, spruces, hemlocks, and 

 others have needlelike leaves. But in gen- 

 eral the green plants have broad, con- 

 spicuous leaves. Leaves vary considerably 

 in size and shape. In the everglades of 

 Florida there grows a fern, the one from 

 which the Boston fern was developed, 

 with a leaf long enough to stretch the 

 length of a large-sized room, 20 feet. 

 One species of pine has needles 1 2 inches 

 long, while the needles of cedar may be 

 less than a quarter of an inch long. 



Parts of a leaf. Many leaves have two 

 distinct parts: a stemlike part called the 

 petiole and a flat, wider part called the 

 blade. There is great variation in leaf 

 blades. They may be narrow and pointed 

 as in the grasses or the common iris; they 

 may be almost round or shield-shaped 

 as in the water lily. They may be smooth 

 or hairy, paper thin or relati\ely thick 

 and stiff. Leaves vary in color, too. When 

 the poinsettia bears its small, inconspic- 

 uous, yellow flowers the upper leaves 

 are not green but bright scarlet. The 

 leaves of the purple beech throughout 

 its whole existence do not appear green. 



Leaves also differ in veining. You read 

 in Unit 1 that leaves may be parallel 

 veined (as in the monocotyledons) or 

 net veined (as in the dicotyledons). And 

 net veining may be of two types as shown 

 in Figures 1 14-1 17, page 85. 



