LECT. IX.] LEAVES. 479 



respiratory organs of plants*. It may be ob- 

 jected to this definition, that some plants, as for 

 example the Dodder, Cuscuta Europea, 'the Sta- 

 pelias, and many of the Cactus tribe, are devoid of 

 leaves ; but in these instances, and in all aphyl- 

 lous plants, the surface of the stem performs the 

 function of the leaves. 



The diversity of character which leaves dis- 

 play is taken advantage of by systematic Botanists 

 for determining species, and consequently every 

 circumstance connected with that diversity, as 

 form, substance, position, attachment., and direction 

 should be made familiar to the student ; as well 

 as the more intimate or internal structure of the 

 leaf itself. In our examination, therefore, as in 

 the case of stems, I shall first demonstrate the 

 external characters of leaves, and then investigate 

 their anatomy or internal structure. 



Let us take any leaf from among those now 

 scattered before us ; this for instance of the Lilac. 



* " Folia transplant et adtrahunt (uti pulmones in anima- 

 " libus) umbramque praebent." Phil. Bot. 81. 



