FIBROVASCULAR TISSUE. 



17 



early stages it is endowed with the vital power in a high de- 

 gree ; but in the progress of development the fibers receive 

 large additions of solid matter, and their density increases until 

 their hardness and rigidity unfit them for vital action, but make 

 them a support for the plant, and prepare them as materials 

 for the use of man. It is more than probable that the woody 

 fiber is capable, at some stages of its existence, of conveying 

 fluids. It often becomes a matter of interest to distinguish 

 different organic fibers which enter so abundantly into so many 

 textile fabrics. This is easily accomplished by the microscope. 



Fig. 16. 



Filaments of Silk. 



Fig. 16 exhibits the appearance of the most common; a repre- 

 sents the magnified filaments of silk, b of flax, c of cotton, d of 

 wool. 



Fibro-vascular Tissue. 



26. This tissue consists of a tube formed by an ex- 

 ternal membrane, with an elastic fiber closely coiled 

 within it, Fig. 17. Sometimes there are several fibers, 

 forming something like a ribbon. 



The above defines the normal form of this tissue, 

 but the variations are numerous, owing to situation 

 and development. This form of tissue may be easily 

 seen by taking a tender branch of the Pokeweed, and 

 cutting one side just through the outer layer, and then 

 bending it so as to make the edges separate, when there 

 will be seen a coil of the spiral vessel connecting the 



Fig. 17. 



Spiral 

 Vessels. 



What takes place by age? How are different fibres distinguished? — 

 26. Of what does the vascular tissue consist ? Where may it be easily seen ? 



