THE TORTION OF LEAVES. 



29 



The cells of parenchyma, 

 w h i c h contain chlorophyll, 

 reach their maximum in species 

 which grow in cool, shady 

 places. 



Fin. 48.— Cros«-section of a blade of Spar- 

 tina junceu, in whicli tlie upper surface Is 

 deeply furrowed. 1 x34.— (Sudwortli.) 



The Tortioii 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 Typha 

 (cat-tail flag) and Gladiolus 

 among endogens ; and some 



species of Liatns (blazing piQ. 49._Cross-section of the central part of a 

 , V -\ l^ blade of Spa7-ti'?ia /u?icea, showing above the mid- 



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

 pied by large cells of parenchyma, destitute of 

 exoffens. chlorophyll. 1x100.— (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, tbe leaves shut up or roll up, leaving the 

 under surface alone exposed. AVhether 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 Agropyriim (quack grass) 

 Secale 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. 



