POSITION 12»j 



may require, and his hypothesis is supported by many 

 plausible arguments. The motion of the trembling 

 Aspen, when every thing around it is quiet and at rest, 

 has caught the attention of many an observing eye. 

 The leaves of this and other species of Poplar, have 

 their petioles so compressed, as to admit of a lateral 

 tremulous motion when exposed to the slightest breath 

 of air. 



We discriminate between the Orange and the Lemon 

 by the winged leaf stalk of the former, and it is not 

 merely a convenient but a necessary mark of distinc- 

 tion. 



The petiole of the Grasses forms an entire sheath, 

 (vagina,) and the late Dr. Muhlenburg in his Grasses 

 of America, has rarely failed to describe it under the 

 latter denomination. 



The leaf-stalks of the common Pea terminate in a 

 tendril, and those of the Traveller's Joy, Clematis Vir- 

 ginica, perform the office of tendrils, by twining like 

 them around their supporters. 



The continuation of the petiole through the leaf is 

 termed its midrib, and its various ramifications, which 

 are most conspicuous on the under surface, constitute 

 the nerves and veins. 



In the examination of leaves we are to learn their 

 different positions and modes of insertion, as well a? 

 the varieties of form, texture, and surface. 



1. POSITION. 



Radical Leaves spring immediately from the root> 



as in the Dandelion and English Cowslip, Fig. 22. 



The leaves of the Stem, sometimes differ from those 



of the Branches, and then it is necessary to distinguish 

 12 



