42 



DIFFERENTIATION OF THE TISSUES 



from their orginal cellulose composition; but they are some- 

 times decidedly thickened and lignifiied. They are short lived; 

 in woody plants the surrounding bundles crowd in and crush 

 them, and in herbaceous plants they often break down and 

 leave a hollow space; in other cases still they may persist for 

 a long time as dead elements. The pith cells are for some time 

 of use in the storage and slow conduction of water and some- 

 times they are employed in the storage of food, even after they 

 are several years old, but they are evidently not commonly 

 depended on long for any essential function. 



The ground meristem lying between the vascular bundles 

 undergoes cell division vertically and transversely and the cells 

 thus formed enlarge and frequently become elongated in the 

 radial direction. In this way the primary 

 medullary rays are formed. The medullary 

 rays have two distinct functions: they carry 

 water and substances in solution radially, 

 inward and outward as needed, and they store 

 water and reserve foods. Since the primary 

 rays extend usually only a few millimeters 

 vertically they are not suited for transporting 

 materials in that direction. A study of cross- 

 sections needs to be supplemented by an ex- 

 amination of vertical tangential sections to 

 make the extent of the medullary rays clear. 

 In these vertical sections it is seen that the 

 primary vascular bundles do not maintain an 

 independent and isolated course through the 

 stem, but anastomose with each other across 

 the upper and lower borders of the rays, as 

 seen in Fig. 20. 

 In the foregoing a type of vascular bundle has been chosen 

 known as the collateral type; where one phloem strand stands 

 radially in front of the xylem strand. While this is the prevailing 

 type, there are others that must not be passed unnoticed here. 

 It sometimes happens that a second phloem strand stands 



Fig. 20. — Diagram 

 showing how the 

 vascular bundles 

 anastomose around 

 the medullary rays. 

 The gaps represent 

 the rays. 



