152 THE ELEMENTS OF VEGETABLE HISTOLOGY 



of primary structure and the order in which they 

 occur, beginning at the outermost layer, are as follows: 



1. Epidermis, 



2. Hypodermis, 



3. Cortical parenchyma, 



4. Endodermis, 



5. Phloem bundles, 



6. Xylem bundles, 



7. Pith parenchyma. 



By reference to Plate 54 it will be. seen that each 

 of these tissues possesses certain structural character- 

 istics and fairly definite cell contents whereby it may 

 be distinguished from the others even when the material 

 is finely powdered. In the identification of unknown 

 samples of roots, it is important to determine whether 

 the material is from a monocotyledonous or a dico- 

 tyledonous plant. It is extremely unlikely that the 

 younger dicotyledonous roots showing primary struc- 

 ture would be present in such material. Therefore 

 the presence of primary tissues in vegetable sub- 

 stances is a fairly certain indication that the material 

 is from a monocotyledonous plant. 



CHARACTERS OF THE PRIMARY ROOT TISSUES 



Epidermis. — The epidermal cells of mature pri- 

 mary roots are usually darker colored than the other 

 tissues. Root hairs or peripheral elongations of the 

 epidermal cells are often present. These structures 

 are integral parts of the epidermal cells and must not 

 be confused with trichomes or plant hairs occurring 

 on over-ground portions of the plant. The root hairs 

 are usually slightly colored, like the epidermal cells, 

 and are extremely thin-walled so as to permit ready 



