PRIMARY AND SECONDARY ROOTS 27 



3. Another point to be noted is that the lateral roots do not 

 arise as up-growths on the surface of the primary root, but come 

 from within it, and are described as endogenous. The slits which 

 they make in the substance of the primary root, and through 

 which they emerge, can be readily seen in a bean seedling 

 {A, Fig. 9, a). A section of a root lengthwise, as at B, shows 

 that the secondary lateral roots are connected with its central 

 more solid core ; the three lowest ones, although they have just 

 begun to grow, have not yet penetrated the outer layer of the 

 root, and would not be visible on the outside of the latter. 



This mode of origin is characteristic of lateral roots generally 

 wherever they are met with. 



Ex. 16. — Germinate and allow broad beans to grow upon damp flannel as 

 in Ex. 3. Watch the development of secondary roots, noting their position 

 and longitudinal rows on the primary root. Slice some and note the endo- 

 genous origin of the secondary roots. 



Very carefully dig up a half-grown mangel, turnip, and carrot ; wash away 

 the soil and note the arrangement of the secondary roots on the primary root. 



Make a deep longitudinal slit with a knife through the ' rind ' down to the 

 ' core ' of a carrot. Split ofif the ' rind ' and examine the ' core ' from whence 

 the secondary roots arise. How many rows of the latter are there ? 



4. Many dicotyledonous plants have roots similar to those of 

 the bean plant. When, as in this case, the primary one continues 

 to grow, keeping distinctly larger than the lateral ones, it is called 

 a tap root. Very good examples are met with among cultivated 

 plants in the carrot, mangel, red clover, and mustard ; in shep- 

 herd's-purse, poppy, and many other weeds, as well as in most 

 broad-leaved trees. 



A number of plants have swollen fleshy roots in which food 

 materials are stored for future use ; they are described as tuberous 

 roots, and must be distinguished from tubers, which are fleshy 

 underground stems. 



To designate the different forms of thickened roots various 

 special terms are in use. The typical carrot root is conical ; that 



