114 



BOTANY FOR BEGINNERS 



CHAP. 



FIG. 1^2. Diagram of growing 

 point of root. RC, root -cap ; 

 K, apex of growing point ; D, 

 calyptrogen ; PR, periblem ; 

 PL, plerome. 



(3.) The calyptrogen (which is another name for dermatogen) 

 forms the piliferous layer and the root-cap. Cells are cut- off 



from the outside of the calyptrogen 

 to form the root-cap. The root-cap 

 by coining in contact with the sharp 

 fragments in the soil protects the 

 growing point from injury. It is 

 worn away as it passes through the 

 soil, and is repaired by the produc- 

 tion of new cells by the calyptrogen. 

 Formation of Branches. 

 The branches of the root are pro- 

 duced from the pericycle, or outer 

 layer of the vascular cylinder. In 

 the stem, as we have seen, the 

 branches arise from the periblem 

 and dermatogen, but in the root 

 the whole of the branch is formed 



from the plerome. The branches of the root are said to be 

 endogenously formed. When the young root is formed it 

 has to force and eat its way through the cortex and the piliferous 

 layer in order to reach the soil. 



SUMMARY. 



A Dicotyledonous Stem. In a dicotyledonous stem the following 

 parts are present, beginning on the outside 



(i) Epidermis ; (2) cortex ; (3) endodermis ; (4) pericycle ; (5) 

 phloem ; (6) cambium ; (7) xylem ; (8) pith. 



Growth in Thickness of a Dicotyledonous Stem. A woody tree 

 shows annual rings of growth which are produced by the activity of the 

 cambium forming new xylem. Each ring consists of a dark coloured 

 and a light coloured portion. The latter is produced in spring, the 

 former in autumn. 



Periderm is formed from the pericycle. The pericycle divides up and 

 forms the phellogen or cork cambium , which forms on the inside a layer 

 of cells which keep their living contents, and an outer ring of cells 

 which lose their living contents. The former is called the phelloderm, 

 and the latter the cork-layer. 



Lenticels are openings found in the periderm. 



A Monocotyledonous Stem differs from a dicotyledonous stem in 

 having the vascular bundles scattered, and in no growth in thickness 

 taking place. 



Boots differ from stems in having alternating masses of phloem and 



