AIR SPACES AND STOMATA 45 



tissue toward the upper side and spongy tissue toward the lower side, 

 so that the two sides of the leaf are very different in their photo- 

 synthetic and other relations, are called diphotophylls (Fig. 15). 

 Those that have palisade tissue on both sides with spongy tissue 

 between, so that the two sides of the leaf are essentially the same, 

 are called diplophylls, meaning double leaves (Fig. 17). The term 

 spongophyll is used for a leaf having spongy parenchyma all the 

 way through (Fig. 16), while leaves that have compact tissue all the 

 way through, whether it is composed of palisade cells or not, are 

 called staurophylls. 



27. Air Spaces and Stomata.— Gaseous exchange between leaves 

 and the atmosphere takes place in connection with three physio- 

 logical processes: respiration, photosynthesis and transpiration. 

 By far the greatest volume of gas (water vapor) is involved in trans- 

 piration and by far the smallest volume in respiration, while photo- 

 synthesis is intermediate in this respect. This interchange of gases 

 is rendered possible through the presence of stomata in the epidermis 

 and intercellular air spaces within the leaf. There is usually a com- 

 paratively large air chamber beneath each stoma and this leads into 

 intercellular spaces of varying sizes that form a connected system of 

 passageways throughout the interior of the leaf. The cell walls that 

 are adjacent to these passages and chambers are thin and are always 

 wet, and the gases diffuse inward and outward through the pro- 

 toplasmic membranes of the cells while in solution in the water. 



The primary cause of the development of air spaces is probably 

 just the opposite of that causing the development of palisade tissue, 

 that is, lack of transpiration. Air spaces are always numerous and 

 comparatively large in the spongy tissue of diphotophylls and they 

 reach their highest development in submerged spongophylls and in 

 the stems and petioles of submerged water plants. On the other 

 hand, air spaces are small and inconspicuous in palisade tissue and 

 are least developed in the staurophylls and succulent leaves of plants 

 of dry and cold regions. 



In submerged water plants the air spaces often exceed the tissues 

 themselves in volume. In some cases they are undoubtedly of con- 

 siderable importance as gas reservoirs. Oxygen is connnonly present 

 in abundance in ordinary lakes, ponds and streams but in swamps 

 and other stagnant bodies of water it is present only in very limited 

 amounts. For this reason, if there were not a supi)lementary supply 

 to draw upon, the plants often would have difficulty in getting 

 enough of this gas for respiration. But the oxygen that is liberated 



