136 THE ROOT 



the xylem by one or more layers of parenchyma. The phloem 

 consists of thin-walled living cells which, as in many plants, are 

 distinguished with difficulty from the surrounding i)arenchyma. 



2. The root of Smilax, Zea or some other monocotyledonous 

 type. Examine earefulh' the trans-section, noting particular!}' 

 the polyarch stele and the central pith-like region. 



3. The origin of lateral roots. Study transverse and longi- 

 sections of the root of the water hyacinth (EichJwrnia crassipes) 

 and observe the method of origin and early ontogeny of the lat- 

 eral root primordia. For comparative jnirposes. make a similar 

 study of lateral root development in bean or willow. 



4. Secotidary growth in roots. Study a series of trans-sections 

 of the root of a woody oi- herbaceous plant cut at levels suc- 

 cessively distal to the region of maturation of the primary vascu- 

 lar system. Observe the method of origin of the cambium and 

 the formation of secondary vascular tissues and the periderm. 



III. Suggested Drawings and Notes. — 



1. Prepare diagrammatic drawings of trans-sections of the 

 root of Banunculus and of some monocotyledonous type showing 

 the position and extent of all the primary tissues. Draw in detail 

 a portion of the stele in each type to illustrate the structure of 

 the primary phloem and xylem. 



2. Prepare diagrams based upon both transverse and longi- 

 sections to illu.strate the origin of lateral roots and their emer- 

 gence to the surface of the mother root. 



3. Prepare a series of diagrams based upon material studied 

 in the laboratory to illustrate the origin of the cambium and the 

 effects of secondary growth upon the primary tissues of the root. 



REFERENCES 



1. Arber. A., The Interpretation of Tioaf and Root in the Angio- 

 sperms. Biol. Kev. K; :81-10:). 1941. 



2. Arnold. C. A.. A Note on the Origin of the Lateral Kootlets 

 of Eichhornia crassipes (]\Iart.) Solms. Amer. Jour. Bot. 

 27 :728-73(). 1940. 



3. De Bary, pp. 348-360. 



4. Eames and MacDaniels, Ch. X. 



