ASCENDING PART. HERBAGE. 9 



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 of 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, 

 whose stems are usually very succulent ; such as 

 Salicornia, Cuscata, Stapelia. 



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). 



