VASCULAR TISSUE. 



vt 



27. It is this form of tissue tliat gives strength to vegeta- 

 bles. Witlioiit it the stems of trees would be unal)le to bear 

 th«'ir own weight, much less could tlu\v he used, as lliry now 

 are, as materials of great strength. Tlie branches of the oak, 

 or hickory, destitute of the woody fibre, would break as easily 

 as the mushrDom. Besides forming a part of the wood, it is 

 found in the bark and midrib of leaves. It protects other, 

 and more delicate portions, and gives form to the plant, ap- 

 pearing to occupy the same place in the vegetable economy 

 as that of bones in the animal. In its early stages it is en- 

 dowed with the vital power in a high degree ; but in the pro- 

 gress of development the fibres receive large additions of so- 

 lid matter, and their densitv increases until their hardness and 

 rigidity unfit them for vital action, but make them a support 

 tor the plant, and prepare them as materials, for the use of 

 man. It is more than probable that the woody fibre is capa- 

 ble, at some stages of its existence, of conveying fluids, but 

 the minuteness of the fibres has as yet presented an insupera- 

 ble dithculty in determininf^ >yhether the fluid nasses between 

 iiitj iibrcs or through them. 



Section 4. — Vascular Tissue. 



28. This tissue consists of a tube formed by an external 

 membrane, with an elastic fibre closely coiled within it, (fig. 

 a 12 1, 12 <>•) Sometimes there are two fibres coiled 

 in opposite directions, as seen in fig. 12, h ; at 

 others there are several 

 fibres forming something i a 

 like a ribbon, as seen in 

 fig. 13, a. 



The above defines the 

 normal form of this tissue, 

 but the variations are nu- 

 merous, owing to situation 

 and development. This 

 ■_| form of tissue may be ea- 

 sily seen by taking; a ten- 

 der branch of the Poke- 

 weed, and cutting one side 

 just through the outer lay- 

 er, and then l>ending it so as to make the 

 edges separate, and there will bo seen a 

 coil of the spiral vessel connecting the two surfaces. Other 

 young branches will answer the same purpose, some etjually 





