ASCENDING PART. HERBAGE. 9 



senting its upper surface to the light, the under 

 commonly differing in hue, and in kind or degree 

 of roughness. The inside is pulpy and vascular. 



31. Leaves receive the sap from the wood by one set 

 of vessels, and expose it to the action oi air, light 

 and heat by their upper surface, while what is su- 

 perfluous passes off by the under. The Sap thus 

 changed assumes peculiar flavours, odours, and other 

 qualities, and is sent by another set of vessels into 

 the bark, to which it adds a new layer every year 

 internally, and another layer to the external part of 

 the wood. Hence the concentric circles in trees, 

 the number of which shows their age, and the breadth 

 of each circle, the abundance and vigour of the 

 foliage which formed it. 



32. Leaves are wanting in some tribes of plants, wh 3se 

 stems are usually very succulent ; such as Salicornia, 

 Cuscuta, Stapdia. 



33. The situation of Leaves (30) is either at the root, 

 or on the stem or branches ; alternate, scattered, 

 opposite, crowded, whorled (3, 4, or more in a 

 whorl), or tufted. 



34. Their position is either close-pressed to the stem, 

 imbricated, erect, spreading, horizontal, reclinate, 

 recurved, or inflexed ; oblique (or twisted) or re- 

 versed (the upper surface turned downward) ; de- 

 pressed, floating, or immersed; two-ranked (spread- 

 ing two ways 14:7); decussated (crossing each 

 other in pairs) ; or unilateral (leaning all to one side). 



