44 



Elementary Botany 



5. A ring of cambium, also interrupted by the passage of 

 the medullary rays. 



6. An interrupted ring of xylem or wood. 



7. A central pith or medulla. 



8. The medullary rays, uniting the pith with the middle 

 bark. (See fig. 65.) 



In the monocotyledonous stem (fig. 67) there is quite a dif- 

 ferent arrangement. On the exterior there is no differentiated 



bark ; within, no separation into 

 pith and medullary rays ; but a 

 number of bundles of procambium 

 scattered amongst the general 

 ground or fundamental tissue. 

 These bundles differ in their de- 

 velopment as well as in their 

 arrangement from those of the 

 dicotyledonous stem. Instead of 

 leaving a layer of vitally active 

 cambium, they are entirely con- 

 verted into xylem and phloem. 



Dicotyledonous bundles which 

 contain cambium are spoken of 

 as open, whilst those bundles which are destitute of this form- 

 ative tissue, as those of monocotyledons, are known as closed. 



We must now note a little more fully the structure and 

 functions of the various parts of the dicotyledonous stem. 



I. The medulla or pith consists entirely of parenchymatous 

 cells, generally dodecahedral in shape, and it forms a cylindrical 

 axis at or towards the centre of the stem. In the earlier stages 

 the cells usually contain a little chlorophyll, and are filled with 

 nutrient substances ; later on they become dry and colourless, 

 and filled with air, and no longer serve any purpose in the life 

 of the plant, so that the stem may be hollowed, all the pith 

 having disappeared, and yet the plant may be living vigorously. 

 The amount of pith varies much in -different plants. In 

 hard- wooded plants, as the Ebony, it is very small ; whilst in 

 soft-wooded plants, as the Elder, it is much larger. Again, 

 we often find in many rapidly growing herbaceous plants, as 



Fig. 67. — Dia^ammatic representation 

 of the distribution of the fibro-vas- 



cular bundles in the 

 section of a Palm stem. 



transverse 



