TISSUE-SYSTEMS 



351 



they lie side by side alternately in a circle round the axis (.AY/- 736). 

 They are always separated by interfascicular ground tissue. 

 A variety, which leads to an apparently very complicated struc- 

 ture in the adult form, is found in the stems of LijcopocUum 

 and some allied plants, where the separate bundles ultimately 

 become gamodesmic {Jig- 756). 



The longitudinal course of the bundles in the stele varies 

 very much in different stems, and will be best described when 

 dealing with the structure of the latter. 



In some cases they can be traced up to the pleroiue, beyond 



Fig. 737. 



/>' « s' u^' p w p 



Fig. 737. Kadial vertical section tlirougli a collateral ftbro-vascular buuJle 

 from the stem of the Sunflower, p'. Pith, .s-, s'. Spiral vessels {Frotoxij- 

 lan). w',ic. Wood-cells. jj,p. Pitted vessels, c. Cambium. a7, .v/. Sieve 

 tubes, p/i. Fibres of the pericycle. ». Jauudlc-shcath. c'. Cortex. After 

 Pl-antl. 



the insertion of the leaves, when the bundles are called canlinc. 

 In other cases they bend outwards and end in the young leaves. 

 Being common to stem and leaf, they are known as common 

 bundles. 



Differentiation of the Bundles. 



The transformation of the procambium merismatic cells into 

 the permanent tissue of the vascular bundle does not take place 

 all at once, but begins at definite spots and extends in a regular 

 manner froni those points. In the first type of collateral bundle 



