NUTRITIVE TISSUES 65 



95. In addition to the conducting and supporting 

 systems, the remainder of the plant serves various 

 functions. Thus, a large portion of green plants con- 

 sists of nutritive tissues. These are usually found in 

 leaves, but are also present in the younger parts of stems. 

 In leaves we can usually distinguish, underneath the 

 upper epidermis, one or more rows of closely packed 

 cells, with their long axes perpendicular to the surface 

 of the leaf, forming the so-called pali- 

 sade parenchyma. In leaves equally 

 lighted on both sides, this palisade 

 parenchyma is dften formed on both 

 surfaces. Below the palisade layers 

 the assimilative cells are looser, form- 

 ing the "sponge" parenchyma, with fiq. 32.— Section of 

 larger intercellular spaces between them, ^ "^ " 

 which connect with the exterior through the stomata. 



96. The system of intercellular spaces is quite marked 

 in all higher plants. These passages are usually con- 



Fjg. 33. — Large intercellular spaces in water-lily petiole, and rush stem. 



tinuous through the petioles of the leaves into the stems 

 and down into the roots. In plants growing in swampy 

 places or in water these intercellular spaces are very 

 much enlarged and apparently serve the double function 

 of providing an ample air supply to the submerged por- 



