Types of vascular Bundles,Radial,Conjoint,Collateral,Bicollateral,Concentric,Cucurbitaceae and Solanaceae, Amphicribral, Quatic Angiosperms and Pteridophytes

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Types of vascular Bundles
According to arrangement of xylem and pholem vascular bundles are of two types
1. Radial
Here xylem and pholem occur in separate bundles and lie on different radii alternating with each other. These are characteristics of roots.

Vascular Bundles - Radial

A - Xylem

B - Pholem

2. Conjoint
Here xylem and pholem combine in one bundle and thus lie on one radius. They are further of three types:

(a) Collateral
When xylem lies inwards facing pith and pholem lies outwards facing the cortex, the vascular bundle is collateral. In gymnosperms and dicot stem cambium is present in between xylem and pholem such a vascular bundles is called open. The vascular bundles of monocots do not have cambium and are called closed. Fig 5.1-45 (a)

(b) Bicollateral
These have pholem on both sides of xylem. Usually a strip of cambium is present outside and inside of the xylem. They are characteristic of family Cucurbitaceae and Solanaceae. Fig. 5.1-45 (b)

Collateral Open - Collateral Closed	Biollateral
Amphicribral	Amphivasal

(c) Concentric
Here one kind of vascular tissue is surrounded by other. Cambial strip is always absent. Fig.5.1-45 (c) It is of two types:
(i) Amphicribral. when xylem lie in centre surrounded by pholem. In aquatic angiosperms and pteridophytes.
(ii) Amphivasal. Here pholem is surrounded by xylem. It is seen in some monocots.