44 THE ECOLOGY OF LEAVES 



shade leaves from the north side of the tree near the base where they 

 are protected from the sun during practically the whole day. 



The fact that palisade tissue is best developed in leaves that are 

 fully exposed to the sun has led to the belief that the significance of 

 the palisade shape of cells is concerned with the protection of the 

 chloroplasts from intense light rays. Although chlorophyll is pro- 

 duced only in light and must have light in order to function, yet light 

 that is too intense destroys chlorophyll by causing its disintegration. 

 In a leaf that has been exposed to direct sunlight for some time the 

 majority of the chloroplasts are found to be arranged along the walls 

 that are perpendicular to the surface where they have a measure of 

 protection from the direct rays of light. On the other hand, a leaf 

 from the same species, but which has been for some time in diffuse 

 light, usually shows many chloroplasts along the w^alls that are 

 parallel to the surface where they may receive a maximum amount 

 of the incident light. A similar response to varying intensities of 

 light is seen in the alga, Mougeotia, in which there is a single rec- 

 tangular, plate-like chloroplast in each cell. ^Yhile this plant is in 

 diffuse light the chloroplast of each cell ordinarily presents its broad 

 side to the direction of the incident light but when the plant is placed 

 in direct sunlight the chloroplast makes a half turn so that its edge 

 is toward the light. 



Although the evidence is strong that light is a factor in the de- 

 velopment of palisade tissue yet its influence in this respect has 

 probably often been overemphasized. When a plant is fully 

 exposed to light it is also exposed to rapid transpiration. It has 

 already been shown that palisade development reaches its maximum 

 in dry land plants where transpiration, in proportion to the rate of 

 absorption, is most rapid. The probability that transpiration is the 

 principal cause of palisade development is also shown by many water 

 plants that have both submerged and aerial leaves. In these cases 

 there is an abrupt change in the degree of palisade development at 

 the sinface of the water, the aerial leaves having much palisade tissue 

 and the submerged leaves none. The change in light intensity here 

 is, of course, not abrupt while the change in transpiration rate is. 

 Here, therefore, as in so many other cases, water appears to be the 

 most important factor. 



26. Structure as a Basis for Classifying Leaves.— Broad leaves, 

 as opposed to needle-shaped leaves and reduced or scale-like leaves, 

 are often classified into four groups on the basis of the structural 

 differences discussed in paragraph 24. Leaves which have palisade 



