becomes greater. Woody stems also tend to 

 develop a "corky" layer outside the cortex 

 proper, and this cork replaces the epidermis 

 (Fig. 13-14). Moreover, the thickness of the 

 bark does not increase indefinitely because 

 the outer layers are continually worn away 

 by the action of the elements. 



Fig. 13-15. Photomicrograph of a cross section of the 

 wood of elm (U/mus americana). The light circles, both 

 large and small, are ducts, between which lie chiefly 

 wood fibers; and the vertical streaks are rays. 



Conduction in the Stem. The upward flow of 

 sap in the xylem, which carries water and 

 salts to the higher parts of the plant, can be 

 demonstrated by girdling experiments. If 

 a cut is made through the cortex and phloem, 

 encircling the stem just to the depth of the 

 cambium, the leaves of the plant remain 

 turgid and do not wilt for a number of days. 

 This shows that water passes the girdled re- 

 gion and reaches the foliage in adequate 

 amounts. However, growth in the stem and 

 roots below the cut stops as soon as the local 

 stores of organic substances are exhausted. 

 Finally the roots begin to die, and then the 

 whole plant dies, indicating that the roots 

 and other nonleafy parts of the plant depend 

 upon synthesized organic substances brought 

 downward from the leaves via the sieve tubes 

 of the phloem. 



Internal girdling, which is the opposite 

 experiment, involves cutting the xylem of a 

 stem without disturbing the phloem. This 

 operation is difficult, but it can be done in 

 small stems, by means of special instruments. 

 The leaves of such a girdled plant begin to 

 wilt and die immediately after the operation, 



Nutrition of Multicellular Plants - 249 



because water cannot get to the leaves to re- 

 place the evaporative losses. And without 

 leaves the whole plant will die, unless it can 

 put forth new foliage from the stem below 

 the level of the operation. 



The Leaf and Its Function. The broad 

 bladelike form of typical leaves is well 

 adapted to their main function, which is to 

 carry on photosynthesis for the plant as a 

 whole. To fulfill this function the leaf must 

 possess an adequate surface that is exposed 

 as directly as possible to sunlight. 



The leaf in dicotyledonous plants consists 

 of a stalk, called the petiole, and a broad 

 part, the blade, which may be either simple 

 or subdivided into leaflets. In structure the 

 petiole closely resembles the stem; and the 

 vascular bundles of the petiole extend out 

 into the blade, forming the midrib of the 

 leaf. In the blade the vascular bundles of 

 the midrib "fan out," forming a network of 

 veins throughout all the blade. Essentially 

 each vein is a small vascular bundle, sur- 

 rounded by a sheath of parenchyma cells. 



Microscopic Structure of a Leaf. The internal 

 structure of the leaf is seen more clearly in 

 cross section (Figs. 13-16 and 13-17). The 

 colorless cells of the upper and lower epi- 

 dermis possess relatively thick, thoroughly 

 cutinized outer cell walls. This epidermal 

 layer protects the more delicate internal cells 

 from drying and from mechanical injuries 

 and infections. The epidermis, especially on 

 the lower side of the leaf, is characterized by 

 the presence of numerous small pores, the 

 stomata. Each stoma is flanked by a pair of 

 guard cells (Figs. 13-16 and 13-17), which 

 control the escape of water vapor, and regu- 

 late the exchange of C0 2 and 2 between 

 the internal tissues and the atmosphere. Un- 

 like epidermal cells, the guard cells possess 

 chloroplasts. 



Most of the space between the upper and 

 lower epidermis is filled with chlorenchyma 

 tissue, but these green cells are loosely ar- 

 ranged, especially in the lower layers (Fig. 

 13-16). Accordingly there is an extensive sys- 

 tem of intercellular air spaces inside the leaf, 



