Vegetable Organography and Physiology. 53 



still exists in regard to the structure and functions of these organs 

 in celhilar plants. We shall therefore select a subdivision of the 

 former class of plants, in describing the anatomy of their organs, 

 and in tracing the analogy which exists between the vegetable 

 and animal kingdoms. 



Naturalists have discovered that there are two orders of plants 

 belonging to vasculares, which are widely different in their ana- 

 tomical structure, and in the laws which govern their develop- 

 ment. These divisions have been termed endogenous and exo- 

 genous. 



Endogenous plants are cylindrical, and are destitute of bark. 

 Their development is by an annular deposition of new ligneous 

 matter loithin the cylinder. The palm, the cane, corn, and the 

 various grasses, are examples of endogenous plants. Exogena? 

 compose a much larger, and a more interesting class. They in- 

 clude all the trees of our forests, for the protection of whose exter- 

 nal surface nature has provided a bark. The reason for this pro- 

 vision will appear, when it is known that the growth of Irees de- 

 pends upon a deposition of new materials upon the outside of the 

 wood, and between it and the bark. 



All exogenous plants, of which class the oak, the elm, the pine, 

 the beach, &c. are examples, are composed of five principal 

 parts ; the medulla or pith, the medullary sheath, the wood, the 

 bark, and the medullary rays. 



It has been said that only four primary tissues enter into the 

 organization of vegetable structure, viz. the cellular tissue, the 

 woody fibre, the spiral vessels, and the ducts. The pith, which 

 is the central portion of the plant, is composed of vesicles of the 

 cellular tissue ; neither spiral vessels or woody fibre enter into 

 its composition. The vesicles of the medulla are slightly bound 

 together ; and their walls, according to the opinion of some phys- 

 iologists, are covered with minute globular bodies, which are re- 

 garded as the nervous organs of the plant.* 



No part of the ascending sap passes through the pith ; but its 

 cells are filled with an elaborated, nutrient fluid, which being 

 disseminated in spring-time, serves to nourish the early buds, until 

 they are sufficiently developed to procure nourishment for them- 

 selves. How far the pith may be considered as the special seat 



* Recherch. Anat. et Physiol, sur la Struct. «fcc. 



