18 



OEGANOGEAPHY AND GLOSSOLOGY. 



The stem bearing opposite branches is dichotom,ous (c. dichotomus, Lamb's Lettuce) ; 

 and trichotomous (c. trichotomus, Oleander), when it continually forks or trifurcates to 

 the extremities of its branches. 



THE LEAVES. 



Leaves (folia) are usually flat, green, horizontal expansions, arising from the 

 nodes, and are the result of the spreading out of a bundle of fibres, the interstices 

 between which are filled with parenchyma. The point of the stem constituting 

 the base of the leaf, and of wJiich the latter is a con- 

 tinuation, forms a small swelling (pulvinus, fig. 54, c), 

 which, when the leaf has fallen, is clearly indicated by a 

 scar (f). 



54. Glycine. 

 Branch, showing the buds 55. Orange, 



after the leaveshave fallen. Leaf with a winged petiole. 



67. Ranunculus. 

 Leaf with amplexicaul petiole. 



56. Acacia heterophyllus. 

 Phyllode. 



The leaves and roots are the principal organs of nutrition, absorbing from 

 the atmosphere gases and liquids suited for the nutrition of the vegetable : they 

 also act as respirators, and as exhalers of useless matters ; and it is in their tissues 

 that the sap, absorbed by the root, and conducted upwards by the stem, parts with 

 its surplus fluids, and acquires all its nutritious properties. 



Of all plant-organs, the leaves are those which present the greatest variety, and 

 which supply most specific characters. 



When the vascular bundle which enters the leaf is prolonged for a certain 

 length before branching to form the skeleton of the blade (limbus), it takes the name 

 ■ of petiole ['petiolus), and the leaf is called peitoZaie (f. petiolatum, Cherry, fig. 6); — 

 when it expands immediately after leaving the node, the leaf is reduced to its blade, 

 and is called sessile (/. sessile, St. John's Wort). When the blade merely narrows so 

 as to form an obscure petiole, it is called sub-petiolate (/. sub-petiolatuni) . 



The petiole may be cylindric [p. cylindricus) ; longitudinally grooved or channelled 



