Describe two types of conjoint vascular bundles.

Conjoint vascular bundles: When the complex tissue (xylem and phloem) is collectively present as neighbours of each other on the same radius, vascular bundle is called Conjoint vascular bundle.

They are of two types: 

a. Collateral vascular bundle: In this type of vascular bundle, xylem lies inwards and the phloem lies outwards. These bundles may be further of open type (secondary growth takes place) containing cambium in between xylem and phloem and closed type if cambium is not present (secondary growth absent). 

b. Bicollateral vascular bundle: When phloem is present in a vascular bundle on both the sides of xylem and intervening cambium tissue, it is called bicollateral vascular bundle. It is a feature of family Cucurbitaceae.