NATURAL ARRANGEMENT OF PLANTS, 



CLASS I. DICOTYLEDONOUS OR 

 EXOGENOUS PLANTS. 



THIS is the most extensive class in the vegetable king- 

 dom, and derives its name from the fact that the seeds 

 are composed of two, or more, lobes, called cotyledons, 

 which enclose the plumule, or embryo of the future 

 plant. As germination commences, the plumule length- 

 ens downwards into a root, called in its early stage, a 

 radicle : at the same time the upper extremity lengthens 

 into a stem, which is composed of bark, woody fibre, 

 spiral vessels, cellular tissue, and a central column of 

 pith. The stem increases in diameter by deposits 

 beneath the bark, but outside the existing fibre. Hence, 

 the plants belonging to this class are called EXOGENOUS 

 (increasing by additions on the outside). In all trees 

 and shrubs of this class the wood is arranged in con- 

 centric layers, the hardest part being nearest the pith. 

 The leaves are reticulated, or furnished with a network 

 of veins. The /ewers- are furnished with stamens and 

 B 



