THE TORTION OF LEAVES. 



The cells of parenchyma, 

 which contain chlorophyll, 

 reach their maximum in species 

 which grow in cool, shady 



places. 



FIG. 48. Cross-section of a blade of Spar- 

 Una juncea, in which the upper surface is 

 deeply furrowed. 1 x 34.-(Sudworth.) 



The Tortion of Leaves. 



The leaves of most flowering 

 plants quite uniformly turn 

 the upper surface to the light 

 and keep the lower surface 

 in the shade. This rule does 

 not hold good with the 

 grasses nor with quite a num- 

 ber of others, such as Typlia 

 (cat-tail flag) and Gladiolus 

 among endogens ; and some 



species Of Liatris (blazing 



O j.__\ i J.T blade of S'partina juncea, showing above the mid- 



star), and Others among rib, a remarkable enlargement, which is occu- 

 pied by large cells of parenchyma, destitute of 

 CXOgens. chlorophyll. IxlOO. (Sudworth.) 



In half or more of the grasses examined, the whole or a major- 

 ity of the leaves, by a twist of the lower portion of the blade, 

 turn "wrong side up," and expose the "lower side " to the sun- 

 light. In most other cases, we have seen that during the warmest 

 and dryest weather, when the sun's rays are the most trying to 

 the life of the plant, the leaves shut up or roll up, leaving the 

 under surface alone exposed. Whether right side up or wrong 

 side up, the surface most exposed generally possesses the firmer 

 epidermis. 



Young leaves of Phleum pratense (Timothy), several species of 

 Bromus (chess), Triticum (wheat), and Agropyrum (quack grass) 

 tSecale cereale (rye), and others, twist once or more with the sun, 

 or in the direction which they would twist were the sun the 

 cause of tortion. 



. 4 9.-Cro88-secn of the central part of a 



