no BOTANY FOR BEGINNERS CHAP. 



points towards the pith^ in the root it points toward the cortex. 

 The vascular cylinder is surrounded by the pericycle. 

 Outside. 



Piliferous layer with root hairs , . \ 



Cortex (parenchyma cells) . . . . I 



Endodermis (single layer of cells). . \ Transverse section of 



Pericycle (single layer of cells) . . ; Dicotyledonous Root. 



Phloem masses\(In wallflower two, j 



Xylem masses /which alternate) . . / 

 Inside. 



Growth in Thickness of the Root. The roots of dico- 

 tyledonous plants in which the stem increases in thickness 

 themselves also grow in thickness. The growth in thickness 

 of roots depends, as in stems, upon the cambium. The cam- 

 bium, see Fig. 128, passes on the outside of the xylem, and on 

 the inside of the phloem. As growth goes on the structure of 

 the root becomes more and more like the stem, until in an old 

 stem it is very difficult to distinguish the two. Periderm is also 

 formed in a root from the pericycle ; this cuts off the cortex, 

 and the root may be smaller after the second year of growth 

 than during the first year. 



The Structure of a Monocotyledonous Root. In 

 the root of a monocotyledonous plant there is a large central 

 cylinder which contains, as a general rule, a larger number of 

 distinct bundles of wood and bast. In some roots there may be 

 as many as twelve alternating masses of xylem and phloem. 

 The structure is essentially the same, but because the cambium 

 layer is absent, there is no growth in thickness. 



Expf. 103. Select a young root of the Wallflower, and cut a thin 

 transverse section from it, and mount in water. Examine under a low 

 power. Note 



(i) The piliferous layer with its root-hairs. 



(ii) The cortex, which is several layers of cells in thickness. 



(iii) The endodermis, which is a single layer of cells surrounding the 

 pericycle. 



(iv) The pericycle, just within the endodermis. 



(v) The alternating masses of phloem and xylem. 



There are only two vascular bundles present. 



EX.PT. 104. Obtain a bulb of the Hyacinth, and from one of the 

 adventitious roots cut a thin transverse section, mount in water. 

 Examine under a low power. Note 



(i) The central cylinder, which is limited by the endodermis and the 

 pericycle. 



(ii) The very numerous masses of xylem and phloem, which also show 

 the alternating arrangement already seen in the Wallflower. 



