16 ELEMENTARY BOTANY. 



between its particles — tlie roots find entrance and force their 

 way vertically and laterally with but little hindrance. The 

 point of growth and elongation is situated near the tip, which 

 is therefore continually thrust forward. The tissue forming the 

 root-cap, though suffering loss by abrasion against the soil-par- 

 ticles, protects the delicate growing portion, and by the latter 

 it is constantly renewed. In case of contact with a solid par- 

 ticle, growth is not impeded on the side that is free; hence the 

 rootlet, by the continued growth on one side, becomes curved 

 and passes around the obstacle. Darwin showed in his ex- 

 periments with the radicles that " if the tip perceives the air 

 to be moister on one side than on the other it transmits an 

 influence to the upper adjoining part, which bends toward the 

 source of the moisture." Presently the roots contract longi- 

 tudinally — the central portion shortening so much, that folds 

 or irregularities on the surface, or in the cortical portion, may 

 be seen. This contraction, amounting sometimes to ten per 

 cent, of the length, has the same effect as tightening the ropes 

 to a ship's mast, and therefore anchors the plant more securely 

 in its position. 



5. The higher plants, such as the common herbs, shrubs 

 and trees, and the ferns, have true roots, the growing point 

 being covered by a root-cap. But in the Mosses, Liverworts, 

 Lichens, Fungi, and Algse true roots are wanting. Rhizoids 

 (root-hairs) are present in the Mosses and in the Liverworts, 

 and they perform the same functions as true roots. 



6. The primary (first) root often persists and remains con- 

 spicuous instead of being soon lost in branching. In this 

 case it is called the tap-root. It may become enlarged or 

 fleshy as in the Turnip, Carrot, etc. The branches that some- 

 times proceed from the radicle in place of, or accompanying 

 the primary root, are often designated as multiple primary 

 roots though really they are secondary. In the Sweet-potato, 

 Dahlia, etc. they become enlarged, serving, like the tap-root, as 

 reservoirs of plant food ; they are in this case said to be tuber- 

 ous. In Grasses and many other plants the roots are fibrous; 

 that is, numerous and thread-like. 



