`star` Complex Tissues
`star` Xylem
`star` Phloem


● The `color{violet}("complex tissues")` are made of more than one type of `color{violet}("cells")` and these work together as a unit.

● `color{violet}("Xylem")` and `color{violet}("phloem constitute")` the `color{violet}("complex tissues")` in plants.


● `color{brown}("Xylem")` functions as a `color{violet}("conducting tissue")` for water and minerals from roots to the stem and leaves.

● It also provides `color{violet}("mechanical strength")` to the `color{violet}("plant parts.")`

● It is composed of four different kinds of elements, namely, `color{violet}("tracheids, vessels, xylem fibres")` and `color{violet}("xylem parenchyma.")`

● `color{violet}("Gymnosperms")` lack vessels in their `color{violet}("xylem.")`

A. `color{green}("Tracheids:")`

● `color{brown}("Tracheids")` are `color{violet}("elongated")` or `color{violet}("tube")` like `color{violet}("cells")` with thick and `color{violet}("lignified walls")` and `color{violet}("tapering ends.")`

● These are `color{violet}("dead")` and are without `color{violet}("protoplasm.")`

● The `color{violet}("inner layers")` of the `color{violet}("cell walls")` have thickenings which vary in form.

● In `color{violet}("flowering plants, tracheids")` and `color{violet}("vessels")` are the main `color{violet}("water transporting elements.")`

B. `color{green}("Vessels")`:

● `color{brown}("Vessel")` is a `color{violet}("long cylindrical tube")`-like structure made up of many cells called `color{violet}("vessel members,")` each with `color{violet}("lignified walls")` and a large central cavity.

● The `color{violet}("vessel cells")` are also devoid of `color{violet}("protoplasm.")`

● `color{violet}("Vessel members")` are interconnected through perforations in their common walls.

● The presence of `color{violet}("vessels")` is a characteristic feature of `color{violet}("angiosperms.")`

C. `color{green}("Xylem Fibres:")`

`color{brown}("Xylem fibres")` have highly `color{violet}("thickened walls")` and obliterated central lumens.

● These may either be `color{violet}("septate or aseptate.")`

D. `color{green}("Xylem Parenchyma:")`

● `color{brown}("Xylem parenchyma")` cells are`color{violet}(" living")` and `color{violet}("thin-walled, ")` and their cell walls are made up of `color{violet}("cellulose.")`

● They store `color{violet}("food materials")` in the form of starch or fat, and other substances like `color{violet}("tannins.")`

● The `color{violet}("radial conduction")` of water takes place by the ray `color{violet}("parenchymatous cells.")`

E. `color{green}("Types of Primary Xylem:")`

● Primary xylem is of two types – `color{brown}("protoxylem and metaxylem.")`

● The first formed `color{violet}("primary xylem elements")` are called `color{brown}("protoxylem")` and the later formed `color{violet}("primary xylem")` is called `color{brown}("metaxylem.")`

`star` `color{green}("Endarch:")`

● In stems, the `color{violet}("protoxylem")` lies towards the centre (pith) and the `color{violet}("metaxylem")` lies towards the periphery of the organ.

● This type of `color{violet}("primary xylem")` is called `color{brown}("endarch.")`

`star` `color{green}("Exarch:")`

● In `color{violet}("roots, the protoxylem")` lies towards periphery and `color{violet}("metaxylem")` lies towards the centre.

● Such arrangement of `color{violet}("primary xylem")` is called `color{brown}("exarch.")`


● `color{brown}("Phloem ")` `color{violet}("transports food materials")`, usually from `color{violet}("leaves")` to other parts of the plant.

● `color{violet}("Phloem")` in angiosperms is composed of `color{violet}("sieve tube elements, ")` companion cells, phloem parenchyma and phloem fibres.

● Gymnosperms have `color{brown}("albuminous cells")` and `color{brown}("sieve cells.")`

● They lack `color{violet}("sieve tubes")` and `color{violet}("companion cells.")`

A. `color{green}("Sieve Tubes:")`

● `color{brown}("Sieve tube elements")` are also `color{violet}("long, tube")`-like structures, arranged longitudinally and are associated with the `color{violet}("companion cells.")`

● Their end walls are perforated in a `color{violet}("sieve-like manner")` to form the `color{violet}("sieve plates.")`

● A `color{violet}("mature sieve element")` possesses a `color{violet}("peripheral cytoplasm")` and a large vacuole but lacks a nucleus.

The functions of `color{violet}("sieve tubes")` are controlled by the `color{violet}("nucleus of companion")` cells.

B. `color{green}("Companion Cells:")`

● The `color{brown}("companion cells")` are `color{violet}("specialised parenchymatous cells,")` which are closely associated with `color{violet}("sieve tube elements.")`

● The `color{violet}("sieve tube elements")` and `color{violet}("companion cells")` are connected by pit fields present between their common longitudinal walls.

● The `color{brown}("companion cells")` help in maintaining the `color{violet}("pressure gradient")` in the `color{violet}("sieve tubes.")`

C. `color{green}("Phloem parenchyma:")`

● `color{brown}("Phloem parenchyma")` is made up of `color{violet}("elongated, tapering cylindrical cells")` which have dense cytoplasm and `color{violet}("nucleus.")`

● The `color{violet}("cell wall")` is composed of cellulose and has `color{violet}("pits")` through which `color{violet}("plasmodesmatal")` connections exist between the cells.

● The `color{violet}("phloem parenchyma")` stores food material and other substances like resins, latex and mucilage.

● `color{violet}("Phloem parenchyma")` is absent in most of the `color{violet}("monocotyledons.")`

D. `color{green}("Phloem fibres:")`

● `color{brown}("Phloem fibres")` (bast fibres) are made up of `color{violet}("sclerenchymatous cells")`.

● These are generally absent in the `color{violet}("primary phloem")` but are found in the `color{violet}("secondary phloem.")`

● These are much `color{violet}("elongated, unbranched")` and have pointed, needle like apices.

● The `color{violet}("cell wall of phloem fibres")` is quite thick.

● At maturity, these `color{violet}("fibres lose")` their protoplasm and become dead.

● `color{violet}("Phloem fibres")` of jute, flax and hemp are used commercially.

E. `color{green}("Types of Primary Phloem:")`

● The first formed `color{violet}("primary phloem")` consists of `color{violet}("narrow sieve")` tubes and is referred to as `color{brown}("protophloem")` and the later formed `color{violet}("phloem")` has bigger sieve tubes and is referred to as `color{brown}("metaphloem.")`