THE TORSION OF LEAVES. 



The cells of parenchyma, 

 which contain chlorophyll, 

 reach their maximum in siiecics 

 which grow in cool, shady 

 jjlaces. 



Fio. IS.— Cross-section of a blade of Sfiar- 

 tina juncen, in whicli tlie upper surface is 

 deeply furrowed. 1 xai.—(Sud worth.) 



The Torsion of Leaves.— 



The leaves of moet flowering 

 plants quite uniformly turn 

 the upper surface to the light 

 and kecj) the lower surface 

 in the shade. This rule does 

 iiot hold good with the 

 grasses nor with quite a num- 

 ber of others, such as Tijpha 

 (cat-tail flag) and Gladiolus 

 among endogens ; and some 



species of Liatns (blazing p^o. 49.-Cross-8ection of the central part of a 



, \ 1 J.1 Iiliirtcof .»^»rtrff)irt jiniccrt,showin{?abovetlu:nud- 



Starj, and others among rib, a renmrkabU- enlargement, which is occu- 



pii'd by larKc colls of parenchyma, destitute of 

 eXOCrens. 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 \\\},'' and expose the ''lower side " to the sun- 

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

 and (Iryest 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 Phlciimpmtcnse (Timothy), several species of 

 Bromus (chess), Trilicum (wheat), and Af/ropyr on (quack grass) 

 Secale cercah (rye), and others, twist once or more with the sun, 

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

 cause of torsion. 



