240 LEAVES 



The mesophyll of submerged leaves is entirely composed of 

 spongy tissue and air chambers ; there is no palisade at all, 

 and the air chambers may be much larger than the cells 

 which surround them. Such plants, of course, do not need 

 protection from transpiration, but do need some arrange- 

 ment to insure their supply of oxygen ; the air chambers 

 constitute such an arrangement. 



As you have noted, that place at which the plant has 

 its relations with the soil is the region immediately sur- 

 rounding the root-hairs. Similarly, that place at which 

 the plant has its principal relations with the air is the 

 region immediately surrounding the cells of the spongy 

 mesophyll, the region of that "internal atmosphere " which 

 the air in the air spaces composes. The root-hairs are ex- 

 posed directly to the soil ; the mesophyll cells do not appear 

 to be directly exposed to the air, but as a matter of fact they 

 are, since the outside air is continuous through the stomates 

 with the air in the aerating system. These two regions 

 the root-hair region and the air-space region may be 

 regarded together as the two regions in which the rela- 

 tions of the plant to the outside world are principally 

 established. The exchange of liquids goes on actively 

 between root-hairs and the surrounding soil, and the ex- 

 change of gases goes on actively between the mesophyll 

 and the air in the spaces which they border. The root- 

 hairs are the organs of liquid exchanges ; the mesophyll 

 cells are the organs of gas exchanges. 



D. Veins. The veins, in a cross section of a leaf, are 

 found to lie in the spongy mesophyll rather than in the 

 palisade ; they are often in contact, however, with the 

 inner ends of the palisade cells. (See Figure 79.} The 



