THE ROOT FUNCTION AND STRUCTURE 43 



on the inside and outside of an organic (either vegetable 

 or animal) membrane, the liquids tend to mix through the 

 membrane. The law of osmosis is that the most rapid 

 flow is toward the denser solution. The protoplasmic lin- 

 ing of the cell wall is such a membrane. The soil water 

 being a weaker solution than the sap in the roots, the 

 flow is into the root. A strong fertilizer sometimes causes 

 a plant to wither, or " burns it." Explain. 



Structure of Roots. The root that grows from the lower 

 end of the caulicle is the first or primary root. Secondary 

 roots branch from the primary root. Branches of second- 

 ary roots are sometimes called tertiary roots. Do the sec- 

 ondary roots grow from the cortex, or from the central 

 cylinder of the primary root? Trim or peel the cortex 

 from a root and its branches and determine whether the 

 branches still hold to the central cylinder of the main root. 



Internal Structure of Roots. A section of a root shows 

 that it consists of a central cylinder (see Fig. 45) sur- 

 rounded by a layer. This layer is called the cortex. The 

 outer layer of cells in the cortex is called the epidermis, 

 and some of the cells of the epidermis are prolonged 

 and form the delicate root-hairs. The cortex resembles 

 the bark of the stem in its nature. The central cylinder 

 contains many tube-like canals, or " vessels " that convey 

 water and food (Fig. 45). Cut a sweet potato across (also 

 a radish and a turnip) and distinguish the central cylin- 

 der, cortex and epidermis. Notice the hard cap on the tip 

 of roots. Roots differ from stems in having no real pith. 



Microscopic Structure of Roots. Near the end of any 

 young root or shoot the cells are found to differ from each 

 other more or less, according to the distance from the 

 point. This differentiation takes place "in the region just 

 back of the growing point. To study growing points, use 



