168 



STRUCTURAL BOTANY 



alike. Thus the older part of the root is clothed by 

 a complete protective covering, similar to the epidermis 

 of the stem, while the piliferous layer, which formed 

 the original outer surface, disappears. This protective 

 covering, which is called the exodermis (ex in Fig. 65), 

 is found in many dicotyledonous roots also, but 

 in them its functions are very temporary, and it is 

 usually soon replaced by secondary periderm. In 

 most monocotyledonous roots there is no periderm, 

 and here the exodermis continues to be the only 

 protective layer throughout the life of the root. 



The tip of the root is enclosed in a root-cap, the 

 development of which will be described below. 



d. Growing-Points and Mode of Branching 



The growth of the stem follows the same general 



course as that of a Dicoty- 

 ledon. Here also the 

 growing-point, where new 

 tissues and organs are 

 produced, lies at the apex 

 of the stern. In this 

 position we find a meri- 

 stem, wLich at first is 

 made up of quite uniform 

 cells. The description 

 given of the growing- 

 point of a Dicotyledon 

 holds good here with 

 only slight differences 

 (see Fig. 66). The ex- 



Fro. 66. Longitudinal median sec- 

 tions of the growing point of the 

 stem in a Monocotyledon, ep, 

 epidermis; p, periblem; pi, ple- 

 rome ; t, initial cells common to 

 periblem and plerome ; I, young 

 leaves. Magnified about 200. 

 (After Douliot.) 



