362 



RELATION TO ENVIRONMENT. 



apical cell, the root cap being cut off by successive divisions of the outer 

 face, while the primary root tissues are deri\-ed from the three lateral 

 faces. 



Fig. 420. 



Median longitudinal section of the 

 apex of a root of the barley, Hordeum 

 vulgare. fe, calyptrogen; </, dermat- 

 ogen; c, its thickened wall; pr, peri- 

 blem; pi, plcrome; C7t, endodennis; 

 i, intercellular air-space in process of 

 formation; a, cell row destined to fonn 

 a vessel; r, exfoliated cells of the root 

 cap. (After Strasburger.) 



Fig. 421. 



Cross-section of fibrovascular bundle 

 in adventitious root of Ranunculus re- 

 pens. It', pcricycle; 5, four radial plates 

 of xylem; alternating with them are 

 groups of phloem. This is a radial 

 bundle. (After De Bary.) 



Function of the root cap. — The root cap serves an important function in 



protecting the delicate mcristem or cambium at the tip of the root. These 

 cells are, of course, very thin-walled, and while there is not so much danger 

 that they would be injured from dr)'ness, since the soil is usually moist 

 enough to prevent evaporation, they would be liable to injur)' from friction 

 with the rough particles of soil. No similar cap is developed on the end 

 of the stem, but the meristem here is protected by the overlapping bud- 

 scales. One of the most striking illustrations of a root cap may be seen in 

 the case of the Pandanus, or screw-pine, often grown in conservatories (see 

 fig. 447). On the prop roots which ha\-e not yet reached the ground the 

 root cap?; can readily be seen, since they are so large that they fit over the 

 end of the root like a thimble on the linger. 



