Chap. 4 PLANTS PROVIDE FOR THEMSELVES AND THE ANIMALS 63 



abundance of chlorophyll (Fig. 4.8). Here the leaf is greenest and the light 

 falling on it is strongest. These cells are the all important food-makers, the 

 links between the energy of the sun and the living world. The lower layers 

 contain spongy cells of odd shapes and hold less chlorophyll than those of the 

 upper layers. They are loosely packed in clusters with air spaces in between. 

 This region of the leaf provides for the income and circulation of gases and 

 the outgo of water. Extra water is also eliminated in droplets (guttation) 

 from openings at the tips of the veins of grasses, corn and many other plants. 

 In early morning the droplets hang in beautiful symmetry on the edges of the 

 leaves of strawberries and jewelweeds. During the day some water is lost from 

 the leaf and at night moisture in the air condenses on its cool surface. The 

 main supply of water is always from the root. 



Root. The main functions of the root are the anchorage of the plant, the 

 absorption of water and mineral matter, the storage of manufactured food and 

 sometimes of chemicals, e.g., nicotine is produced in the roots of tobacco 

 plants and transported to the leaves. The spread of surface necessary for ab- 

 sorption also makes it an efficient anchor in the soil (Fig. 4.9). The root is 

 the extension of the stem and resembles it in having long tapering branches 

 and an essentially similar structure, although the pattern of the conducting 

 tubes is different. Although roots are various in size, form, and structure, they 

 have no such diversity as the leaves and stems, for conditions in the soil are 

 less variable than those in the air. 



Of all the material which the root absorbs the most important is water. It 

 is a great part of the plant substance and as essential for the processes of living 

 as it is in animals. Absorption occurs exclusively in the microscopic root hairs 

 in the white terminal parts of roots, the ones whose injury in transplanting is 

 followed by the familiar wilting of the plant. Near the tip of each new root, 

 hairs are continually forming, a little farther back they are constantly dying. 

 The root hair is a single cell of the epidermis. It grows outward in a hairlike 

 projection that turns and twists about the particles which in any moist soil are 

 clothed in a thin capillary film of water (Fig. 4.9). The root hair is an osmotic 

 mechanism (Chap. 2, p. 22). Water and salts enter it but sugar does not pass 

 out. Although each root hair is virtually microscopic, their total area is a 

 marvel of expansiveness. In one species of grass the total length of root hairs 



Fig. 4.9. A. The root system of a corn plant. (After Weaver.) B. Diagram of a 

 section of a root tip and its different zones. Cells of the root cap are worn off and 

 replaced by new ones from the growing zone above it. The force that pushes the 

 root through the soil is the lengthening of cells in the elongating zone. Epidermis, 

 root hairs, and the ducts of the food-transporting phloem and the water-transporting 

 xylem all develop in this zone. (After Woodruff & Baitsell: Foundations of Biology, 

 ed. 7. New York, The Macmillan Co., 1951.) C. Root hairs are branches of 

 epidermal cells. In every well-grown root billions of root hairs take in water from the 

 films of it that surround particles of soil wherever there is moisture on the ground. 



