LESSON 9.] VASCULAR TISSUE. 29 



172. This movement of the sap alters the form of the cells 

 through which it passes, and elongates them. 



173. The walls of most of these elongated cells become thickened, 

 they have a tendency to aggregate, and thus is formed, in the midst 

 of the cellular tissue, bundles of elongated cells, or vessels, called 

 vascular bundles, which to the naked eye look like dense fibres 

 running through the tissue of the plant. 



174. In one great division of plants (Monocotyledons], to which 

 the Grasses, Lilies, Palms, &c., belong, the development of these 

 vascular bundles stops short at a certain stage, and they undergo no 

 further alteration. 



175. In another class, on the contrary (Dicotyledons), to which 

 belong our forest trees, kitchen vegetables, and many others, there is 

 a continuous development of cells on the outer side of each vascular 

 bundle, which become in turn vascular bundle cells, and so unceas- 

 ingly increase the thickness of the bundles. 



176. As a consequence, they gradually close up together into a 

 firm tissue, or into that form we call wood. 



177. In relation to human wants, these vascular bundles become 

 important on account of the chief portion of their contents (the spiral 

 fibre) constituting wood, and woody fibre, or bass. 



178. Examined by the microscope, vascular tissue, in its original 

 development, presents itself as a spirally wound coil of woody fibre, 

 within a cell, of cellulose (Fig. 47). PIG 



179. A vascular bundle, microscopically 

 examined, displays vessels in several distinct 

 forms ; some like Fig. 47, others becoming an- 

 nular ; annular vessels perfectly formed ; scal- 

 ariform (scala, a ladder) vessels ; and old ves- 

 sels either emptied of their contents, or filled 

 with woody fibre. 



180. There appears to be great probabil- 

 ity for believing that spiral vessels are always 

 formed originally like Fig. 47 ; having sub- 

 served the purpose for which they were de- 

 veloped, they begin to degenerate ; this process 

 is, however, a slow one, and no great impedi- 

 ment appears to accrue for some time to the 

 transmission of the sap not until the ligneous Spiral vessels - 

 elements are entirely removed, or the old vessels be filled with them. 



