STRUCTURE 239 



ration are both associated with prominent development of 

 palisade. Does it follow from this that palisade tissue 

 protects the plant from too much light or from too much 

 transpiration? Not necessarily. To find the cause which 

 induces a structure is not to prove the advantage of that 

 structure. 



As to light, it has been held that the shape of palisade 

 cells is an advantage in that it reduces the number of 

 walls the light has to penetrate in reaching the chloroplasts, 

 and that it permits the chloroplasts to get out of danger 

 in case of too intense light, that is, they can move to the 

 bottoms of these deep cells. Evidence shows that very 

 intense light does injure chlorophyll, but evidence also 

 shows that in some palisade cells the chloroplasts do not 

 move; in others, however, their movements have been 

 observed and they have been seen to assume various posi- 

 tions, apparently under the influence of light. 



As to reducing the loss of water by transpiration, evi- 

 dently the compactness of the palisade tissue is an advantage 

 as compared with the looseness of the spongy tissue. As 

 in the case of the closure of stomates by the guard-cells, 

 however, it is doubtful whether this structure has a pro- 

 tective effect with respect to water loss which actually 

 amounts to much. 



b. Spongy Mesophyll. The air spaces which characterize 

 the spongy mesophyll deserve our attention as much as the 

 cells of that tissue themselves. These air spaces are con- 

 nected ; they form a continuous system in which air can 

 circulate freely throughout the leaf; it is sometimes 

 called the aerating system. In some plants the air spaces 

 are beautifully symmetrical in arrangement. They are 

 particularly well developed in connection with water plants. 



