THE BOOT. 235 



EXERCISE LXXII. 

 The Minute Structure of Hoots. 



In the germination of dicotyledons the cells of 

 the root-end of the radicle multiply, and the central 

 ones are changed into vessels continuous with those 

 of the stem. The developed root differs from the 

 stem in having neither pith nor medullary rays. 

 Like the stem, its thickness increases by the annual 

 formation of a layer of wood and a layer of bark. 

 Spiral ducts are never found in roots ; such fibres 

 and ducts as enter into its composition are like those 

 found in the stem. Its cells are filled with sap and 

 with starch. In monocotyledons the multitude of 

 fibrous roots, which issue from the side of the radi- 

 cle in germination, are exactly like the stem in their 

 minute structure. 



Roots grow in length by additions of matter at 

 the tip, or free end, while the stem grows throughout 

 its whole length. You may test 

 this statement by marking off into FIG. 404. 



four equal divisions, with ink, the 

 parts of the root of a sprouting pea. 

 After leaving it in the soil for three 

 or four days, observe whether the 

 parts have all lengthened to the 

 same extent. It has been calculated 

 that the growth is confined to a 

 space of about one-sixth of an inch 

 from its tip. 



Fig. 404 represents the struct- 

 ure of the growing extremity of roots and rootlets. 

 The darkened cells (b) are the region of vital activity ; 



