44 COURSE OF THE SAP. 



ter office, and another to the secretion of peculiar fluids 

 in the bark. See Phil. Trans, for 1 801, /;. 337. In the 

 barkprincipally,if I mistake not, the peculiar secretions 

 of the plant are perfected, as gum, resin, &c., each un- 

 doubtedly in an appropriate set of vessels. From 

 what has just been said of the office of leaves, we rea- 

 dily perceive why all the part of a branch above a leaf 

 or leaf-bud dies when cut, as each portion receives 

 nourishment, and the means of increase, from the leaf 

 above it. 



By the above view of the vegetable osconomy, it 

 appears that the vascular system of plants is strictly 

 annual. This, of course, is admitted in herbaceous 

 plants, the existence of whose stems, and often of the 

 whole individual, is limited to one season ; but it is no 

 less true with regard to trees. The layer of alburnum 

 on the one hand is added to the wood, and the libe7\ 

 or inner layer of the bark, is on the other annexed to 

 the layers formed in preceding seasons, and neither 

 has any share in the process of vegetation for the year 

 ensuing. Still, as they continue for a long time to be 

 living bodies, and help to perfect, if not to form, se- 

 cretions, they must receive some portion of nourish- 

 ment from those more active parts which have taken 

 up their late functions. 



There is a tribe of plants called rnonocotyledoneSy 

 having only one lobe to the seed*, whose growth re- 

 quires particular mention. To these belongs the na- 



* Or rather no true cotyledon at all. 



