ix THE HISTOLOGY OF THE SHOOT AND ROOT 115 



xylem. Dicotyledonous roots grow in thickness by a cambium which 

 forms new xylem and phloem. A monocotyledonous root only differs 

 from that of a dicotyledon in having more masses of xylem and phloem 

 and no secondary growth. 



Leaves are outgrowths of the stem, and consist of 



(i) An upper epidermis; (2) palisade tissue; (3) vascular bundles; 

 (4) spongy parenchyma : (5) lower epidermis with stomata. 



Growing Point of Stem. There are three layers of cells in the grow- 

 ing point of the stem, viz., (a) Dermatogen, which forms the epidermis ; 

 (b) Peribkm, which forms the cortex ; (c) Plerome, which forms the 

 vascular bundles and pith. 



Growing Point of Root. In the root there are three layers of cells 

 from which all parts of the root are formed (a) Plerome, which forms 

 the vascular cylinder ; (b) Perible/n, which forms the cortex ; (c) Calyp- 

 trogen, which forms the piliferous layer and root-cap. 



QUESTIONS ON CHAPTER IX. 



(1) Describe the way in which the stem of a dicotyledonous tree 

 grows in thickness, and explain how it is that its wood shows annual 

 rings. (1889.) 



(2) Compare the structure of the stem of a monocotyledonous plant 

 with the structure of the stem of a dicotyledonous plant. 



(3) Describe the structure of an ordinary foliage-leaf as seen in 

 transverse section. 



(4) Explain how periderm is formed. How does periderm differ 

 from bark ? 



(5) What is a medullary ray? What is meant by primary and 

 secondary medullary rays ? 



(6) Describe the structure of the growing point of the stem, and con- 

 trast it with that of the root. (1897.) 



(7) What is cambium ? What is its position in the stem of dicotyle- 

 donous plants, and what is its use ? (1890 T. ) 



(8) Explain exactly how the root and the stem of any dicotyledonous 

 plant differ from each other in structure, as seen in transverse section 

 under the microscope. 



(9) How does the branching of the stem differ from the branching of 

 the root ? 



(10) How can the longitudinal course of the vascular bundles in any 

 stem be determined ? 



(u) What is a lenticel? On what parts of a plant are lenticels 

 found ? 



(12) Describe the structure of the stem in any monocotyledon, as 

 seen in longitudinal and in transverse sections. 



(13) Briefly describe the chief anatomical differences between the 

 stem of a monocotyledon and that of a dicotyledon. (1891 T. ) 



(14) The stem of an oak tree continues to grow in thickness so long 

 as the tree lives, whereas the stem of a palm tree does not grow any 

 thicker when once formed. Explain the cause of this difference. 



I 2 



