250 - Multicellular Plants 



EPIDERMIS 

 ENDODERMIS 

 XYLEM 

 PHLOEM 



STOMA 



GUARD CELL 



EPIDERMIS 



GREEN PARENCHYMA 



and these internal air spaces communicate 

 with the outside air through the stomata. 



The network of veins, or vascular bundles, 

 in the leaf is so extensive that one or more 

 bundles is likely to be present in any section 

 of the leaf, however small. The xvlem of 

 each vein occupies an upper position rela- 

 tive to the phloem, and the smaller veins 

 may consist of only a few ducts and a cor- 

 responding number of sieve tubes. The very 

 smallest veins — out near the margin of the 

 blade — lack sieve tubes and consist of only 

 one or two ducts, which end blindly among 



Fig. 13-16. Cross section of portion of the blade 

 of a leaf, including a single vein. 



the chlorenchyma cells. As in the stem, the 

 vascular bundles of the leaf are chiefly chan- 

 nels of distribution. Water and salts are 

 brought to the chlorenchyma cells by the 

 ducts, and organic substances are carried out 

 of the leaf by the sieve tubes. 



Transpiration. On sunny days the leaves give 

 olf large amounts of water vapor, provided 

 the plant is able to absorb a compensating 

 quantity of water from the soil. This loss of 

 water vapor from the leaves, which is con- 

 trolled by the guard cells, is called trans- 

 piration. The heat of the sun keeps vaporiz- 



Fig. 13-17. Photograph of a cross section of a lilac leaf. One large vein lies in 

 the thick part of the section, and two very small veins are present in the thinner 

 part to the left. 



