172 PLANT PHYSIOLOGY AND ECOLOGY 



extensible, and it surrounds a fluid and semi-fluid mass. The 

 shape of the ceU is in consequence very easily changed. The 

 movement of the chloroplasts into lines or rows throughout the 

 cell doubtless causes it to elongate in the direction of the rows. 

 In the sun the cell thus becomes lengthened in line with the light 

 rays, and perpendicularly to the surface of the leaf. It becomes 

 a vertical palisade cell, and is termed jyrolate, since it is drawn out 

 in the direction of the poles. In the shade the cell is elongated 

 at right angles to the light ray, and parallel to the surface. It 

 develops into a horizontal sponge cell, which is flattened contrary 

 to the poles, and is hence termed oblate. The form of the sponge 

 cell is further modified by the development of air spaces. The 

 chloroplasts of some species, especially monocotyledons, do not 

 appear to respond to varying light intensities by arranging them- 

 selves in rows. In such j^lants all the cells of the leaf remain 

 more or less globose, and there is no distinction into palisade 

 and sponge. 



The palisade cell is the normal result of the response of the 

 chloroplasts to sunlight. The sponge cell is due to the action 

 of diffuse light, or shade, upon the chloroplasts. Palisade cells 

 are usually converted into sponge cells in the shade, and sponge 

 into palisade in the sun. The latter is illustrated by leaves more 

 or less illuminated from below, in which palisade tissue appears 

 on both sides. The formation of sponge tissue in diffuse light 

 is a characteristic result when plants grow in the shade of others. 

 This is equally true of leaves deeply shaded by others of the same 

 plant, as is the case in trees and shrubs, and of those which grow 

 in the diffuse light of ponds or other bodies of water. The upper 

 half of a horizontal leaf shades the lower, producing the typical 

 differentiation of the leaf into palisade and sponge. A thick 

 covering of hairs shades the palisade tissue, converting it into 

 sponge tissue. 



iQi. Modifications of the chlorenchym. The conversion of the 

 chlorenchym into the two tissues, palisade and sponge, is the 

 direct result of the unequal illumination of the leaf surfaces. 

 This is the normal occurrence in the usual horizontal type of 

 leaf. Exceptions occur only in the monocotyledons already 

 noted, in which the leaf tissue consists throughout of sponge-like 

 cells. In certain stable species, also, which are now found in 

 diffuse light, the palisade tissue has not been developed by the 



