Primary permanent tissue may be simple and complex. Simple tissue is made up of one type of cells and complex tissue is made up of more than one type of cells working together as a unit. To these may be added another kind of tissue the secretory tissue.
Complex tissue consists of xylem or wood which is conducting tissue and composed of elements of different kinds like tracheids, vessels or tracheae, wood fibres and wood parenchyma. Xylem is meant to conduct water and mineral salts from root to leaf and give mechanical strength to the plant body. Except wood parenchyma all other xylem elements are lignified, thick walled and dead. Phloem or bast is another conducting tissue and is composed of sieve tubes companion cells, Phloem parenchyma and bast fibres. Phloem is meant to conduct prepared food material from leaf to storage organs and the growing regions.
According to arrangement of xylem and phloem vascular bundles are Radial, Conjoint and collateral.
Bi-collateral vascular bundle:
When in a collateral bundle both phloem and cambium occur twice once on outer side of xylem and then again on its inner side. The sequence is outer phloem, outer cambium, xylem, inner cambium and inner phloem. Bi-collateral bundle is characteristic of the gourd family. It is always open.
In case of young Gourd (Cucurbita) stem vascular bundles are bi-collateral usually ten in numbers and are arranged in two ways. Each bundle consists of (a) xylem (b) two strips of cambium and (c) two patches of phloem.
(a) Xylem occupies the centre of the bundle and consists on outer side, of very wide vessels (pitted). Which constitute the metaxylem and on inner side of narrower vessels which constitute protoxylem. Protoxylem vessels remain scattered. There may be some tracheids and wood fibres but wood parenchyma is abundant.
(b) Cambium
This tissue occurs in two strips the outer and the inner one on each side of xylem. Its cells are thin walled and rectangular and arranged in radial rows. Outer cambium is many layered and is more or less flat. While the inner cambium is few layered and curved. Each strip of cambium gradually merges into phloem and xylem.
(c) Phloem
It occurs in tow patches. The outer and the inner, outer phloem is plano-convex and inner one semilunar in shape. Each patch of phloem consists of sieve tubes, companion cells and phloem parenchyma. Sieve tubes are very conspicuous in the phloem of the cucurbita stem. Here and there sieve plates with perforations in them may be distinctly seen. The rest of the phloem is made up of small thin walled cells which constitute phloem parenchyma.
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