[Chap. IX ENVIRONMENT AND LEAF DEVELOPMENT 81 



hormones are formed in the blades in hght, and their unequal distribu- 

 tion in the petiole results in a greater lengthening of the cells on the 

 shaded side. 



When an environmental factor is changed, all the tissues of the leaf 

 may not be afiFected equally in their development. The enlargement or 

 the differentiation of one tissue may be increased or decreased more 

 than that of others. If the growth of the 

 epidermis is checked sooner than that of 

 the mesophyll, the mesophyll cells will 

 be closer together and the air spaces 

 smaller. If the growth of mesophyll cells 

 is checked sooner than that of the epi- 

 dermis, the leaf will have larger air 

 spaces. Air-space differences usually re- 

 sult in either an increase or a decrease of 

 the area of mesophyll wall surface ex- 

 posed to the air spaces. 



If the edge of a blade matures before 

 growth stops in the middle, stresses de- 

 velop within the leaf and it may become 

 convex or curled. The rigidity of manv 

 large leaves and the curving and twisting 

 of petioles may be due to such internal 

 stresses and strains between tissues. 



The lengthening, bending, and twist- 

 ing of petioles among adjacent leaves on 

 inclined stems, and on vines growing on 

 walls, often result in an arrangement and 

 spacing of blades which when viewed 

 from the direction of the most intense 

 light appear as "leaf mosaics." Rosettes 

 of plantain and dandelion have similar 

 leaf arrangements. 



The leaves of prickly lettuce and other 

 "compass plants"^ growing in dry, in- 

 tensely lighted habitats are twisted to a 

 more or less vertical position in a north- 



^ Lactuca scariola, L. saligna, Silphium lacinia- 

 tum, and S. terebinthinaceum. 



Fig. 40. When the growing 

 stem tip of the mermaid weed 

 is submerged, the leaves that 

 develop are finely divided; 

 when it is in air, the leaves that 

 develop are not divided. 



