a a 
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, Seals Organography and Physiology. 53 
“still éxists in regard to the structure and functions of these organs 
in cellular 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 within the cylinder. The palm, the cane, corn, and the 
Various grasses, are examples of endogenous plants. Hxogene 
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 trees de- 
pends upon a deposition of new aia apan the outside of the 
wood, and between it and the gs 
All exogenous plants, of which a the oak, the elm, the pine, 
the beach, &c. are examples, are com of 
parts; the medulla or pith, the medullary sheath, the woud; 
bark, and the medullary rays. 
It has been said that only four primary tissues enter into shat 
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; | 
cells are filled with an elaborated, nutrient fluid, which Sar 
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 
ESE EPS MI ME De 
* Recherch. Anat. et Physiol. sur Ja Struct. &c. 
ie * 
