Xll 



CONTENTS 



PAGE 



Section 42. Roots as to origin ........ 125 



A. Primary roots . . ... . . . 125 



B. Secondary roots 126 



C. Branches of roots . . . . . . 128 



Section 43. Structure and growth of roots 129 



A. Root curvature 131 



B. Tropisms of roots ....... 132 



C. Root-cap 134 



D. Root-hairs 135 



E. Cortex and stele 137 



Section 44. Duration of roots. Annuals, biennials, and perennials . 139 



Section 45. Contraction of roots 140 



CHAPTER FIVE: STEMS 



Section 46. Introductory ......... 143 



Section 47. Aerial stems , . 145 



A. Erect stems 146 



B. Prostrate stems 149 



C. Climbing stems 151 



D. Exceptional forms of stems 154 



Section 48. Underground stems 155 



A. Rhizomes 156 



B. Tubers 158 



C. Bulbs . . , 159 



D. Corms 160 



E. Stools 161 



Section 49. The vernal habit 161 



Section 50. The structure and growth of stems .... 162 



A. Contrast with roots 163 



B. Epidermis, cortex, and stele ..... 164 



C. Arrangement of the vascular bundles . . . 165 



D. The cylindrical arrangement ..... 165 



E. The scattered arrangement ..... 171 



F. Cells of the bundles 173 



G. Rings in the wood. Age of trees . . . 176 

 H. The bark and things related to it. Lenticels . 178 



/. Grafting and budding 181 



Section 51. Stems in the classification of seed plants . . . 183 



