52 VEGETABLE PHYSIOLOGY. [LESSON 19. 



throughout the section ; from the outer layer of it, which represents 

 the bark in the Exogen, to the internal portion which, in the same 

 order, would be the pith it is, in fact, only interrupted by the vas- 

 cular bundles. 



304. The dotted vessels are seen at &, b, b, b ; c, c, c, c, are the 

 woody fibres, and d, d, d, d, the spiral vessels. 



305. From the peculiar mode of growth of the Endogenous stems 

 they have perfect immunity from the effects resulting to Exogenous 

 stems, of the parasitic plants which cling to, and climb around them. 



306. In the Endogenous stem the soft part is internal, whereby 

 the outer portion, from its greater density, is enabled to resist the 

 pressure of the climber; but in the Exogenous stem the soft wood is 

 external, and consequently yields to the least pressure, and climbing 

 plants, therefore, make deep and permanent indentations in the 

 latter. 



307. Some Endogenous stems grow with such rapidity, that the 

 sudden increase of the outer part occasions a rupture of the central 

 cells, and by this means the hollow in the stems of grasses is pro- 

 duced. 



308. The vascular bundles, in the Bamboo, cross the stem, and 

 form a series of partitions which divide the cane, and give it a jointed 

 appearance ; between these partitions, however, the central cells have 

 been ruptured, in the manner above alluded to. 



LESSON XIX. 



THE EXOGENOUS STEM. 



309. Like the Endogenous, the Exogenous stem in its earliest 

 condition is wholly cellular. 



310. By degrees, and as the plant increases in age, the cellular 

 tissue begins to be traversed by vascular bundles, which soon di- 

 vide the stem into two marked portions ; one of these forming the 

 central pith, or medulla ; and the other forming the cortical bark, 

 covered with epidermis. The connection between these two portions 

 (pith and bark) is maintained by lines of cellular tissue, called me- 

 dullary rays, which are interposed between the vascular bundles. 



311. The complete structure of mature exogenous stems, which 



