THE TISSUE SYSTEMS 283 



plants which contain a milky liquid is called latiriferous 

 tissue. There is no fixed type for the vessels that carry 

 this fluid, as they vary greatly in different plants, being 

 simple in the asclepias (milk-weed), and complex in the 

 dandelion. 



481. Tissue Systems. The parts of complex plants 

 may be conveniently grouped into three tissue systems: 



(1) Fibro-vascular tissue system. This is composed of 

 fibro-vascular bundles. The fibrous framework of roots, 

 stems, and leaves is made of fibro-vascular bundles. ("Fibro- 

 vascular" means fibrous or long and slender, and having long 

 openings or channels.) Each bundle is composed of two 

 fundamental parts: phloem and xylem. The bast fibers 

 may or may not be present. Phloem is another name for 

 the soft bast or sieve tissue, while xylem is the name of the 

 lignified or woody part, and is composed chiefly of the wood 

 cells, tracheids, and ducts. In stems of dicotyledons (exogens), 

 these two parts of the bundle are separated by cambium, 

 a meristematic layer giving rise to xylem on one side and to 

 phloem on the other. For types of bundles, see next chapter. 



(2) Fundamental tissue system. This is composed of the 

 parenchymatous tissue already described. The fibrovas- 

 cular system may be said to be imbedded in the funda- 

 mental tissue. (3) Epidermal tissue system. This is the 

 covering of the other systems, and is composed of epidermal 

 tissue, already described. It should be borne in mind that 

 the types of cells and tissues as defined in this chapter 

 are not all that may be found in plants. There are many 

 intermediate forms, e.g., tracheids and ducts blend the one 

 into the other; and the same is true of wood cells and tracheids. 



482. Summary of tissues studied: 



1. Parenchymatous tissue. 



a. meristematic. 



b. permanent. 



2. Epidermal tissue. 



