21 



IT appears from experiments, that leaves perform 

 some office similar to the lungs of animals ; at least, 

 when healthy and exposed to sunshine, that they 

 exhale oxygen gas through the pores on their surface, 

 afterwards described, and at nigju or in cloudy weather 

 that they exhale carbonic acid gas. 



A leaf 1 may be said, with several exceptions, to con- 

 sist of a leaf-stalk 2 , and a leaf-plate, which is the part 

 usually termed the leaf. When the leaf stalk is want- 

 ing, or so short that the base of the leaf touches the 

 branch or stem, it is termed a sitting 3 leaf, as in the 

 poppy. The inner base 4 of the leaf-stalk, where it joins 

 the stem, is the place where buds are formed. The leaf- 

 plate 5 , or proper leaf itself, is sometimes wanting, the 

 leaf-stalk only spreading out like a leaf. In such cases, 

 not the surfaces, which are both alike, are presented to 

 earth and sky, but the edges. 



The leaf-stalk may be simple, compound, round, 

 flattish 6 , channelled 7 , winged 8 , tendrilled 9 , or sheath- 

 ing 10 . 



From the upper end of the leaf-stalk a number of 



(1) In Latin, Folium. 



(2) In Latin, Petiolus, whence Petiolatum. 

 (3) In Latin, Stssile. (4) In Latin, Axilla, whence Axillaris. 



(5) In Latin, Lamina. 



(6) la Latin, Compressus, which is objectionable. 

 (7) In Latin, Canaliculatus. (8) In Latin, Alatus. 



(9) In Latin, Cirriferus- (10) In Latin, Vaginans. 



