WOODY AND VASCULAR TISSUE. 35 



network of which consists of woody fibres inclosing spiral 

 vessels as a protecting sheath, and through the pores of the 

 leaves and young shoots this oxygen escapes into the atmo- 

 sphere. The other kinds of vascular tissue, particularly the 

 porous, convey fluid which they distribute horizontally and 

 longitudinally through the vegetable system. Spiral fibres 

 are abundant in young plants and shoots ; in the hard sterna 

 of trees and shrubs, they chiefly surround the pith. 



37. The cell has now been shown to be the type of all the 

 tissues of plants, and to be the basis of the vegetable struc- 

 ture. To the elongation of cells and to the deposition of 

 thickening layers in their interior, the various kinds of vessels 

 owe their origin. In the lower tribes of plants, such as 

 mosses and algae, the cells multiply in their primitive form, 

 which they retain during the life of the plant ; in vegetation 

 of a higher grade, some of the cells early undergo the transfor- 

 mations already described, and become elongated into tubes of 

 vascular and woody tissue, which tubes, imbedded in the 

 parenchyma or cellular tissue, impart additional strength and 

 solidity to the vegetable structure, conveying fluids and air 

 to every part of it. The beautiful researches of Treviranus 

 and M. Mirbel have proved that these vessels are only a 

 series of superposed vesicles of membranous and fibro-mem- 

 branous cellular tissue, the septa of which have been absorbed 

 in the process of growth. These facts may be verified by 

 direct observation on the young and growing tissues of plants, 

 if those tissues be placed beneath the microscope. All the 

 different forms of tissue which we have described, pass insen- 

 sibly one into the other through intermediate gradations, and 



