COLLATERAL BUNDLES 



357 



the form of flanges which divide the leptome into two or more sections ; 

 by this means the leptome is more effectively protected against com- 

 pression. In the collateral bundles of Dicotyledons and (Jymnosperms 

 the hadrome elements are usually disposed in rows which run parallel 

 to the plane of symmetry of the bundle (Fig. 145). This arrangement 



Fig. 145. 



Collateral bundle from the leaf of Malva sylvestris (T.S.). 



is due to the fact, that a serial cambium is formed, at an early stage of 

 development, which remains active at the junction of hadrome and 

 leptome for a long time, or even indefinitely ; in the latter event the 

 persistent strips of fascicular cambium form the starting points of the 

 secondary cambial cylinder. Monocotyledonous plants, on the other 

 hand, never develop a serial fascicular cambium ; hence there is no 

 persistent cambial zone at the junction of hadrome and leptome in their 

 bundles. In other words, the entire primary procambium is straight- 

 way converted into permanent tissue. 



As already explained, an intercellular passage of varying width 

 occurs in each hadrome-straud in many Monocotyledons (e.g. Acorus 

 Calamus, Butomus, Sagittaria, Alisma, Cypekaceae, Juncaceae, etc.), and 



