Phylogeny of Psilotum / why They are simple pteridophytes / unique FEATURES OF THE PSILOTALES

unique features of the psilotales / PSILOTUM

Why is this small order, the Psilotales, so unique among vascular plants.

(a) The Psilotales are an order showing essentially primitive features. The plants are rootless; the development from the zygote shows that this condition is genuinely primitive and not a result of reduction due to specialization of habit. 

The plant body is relatively unspecialized anatomically.

 The sporophyte is in many respects like a primitive simple thallus, the main continuous axis weakly differentiated into underground rhizome and leafy aerial shoot. 

The gametophyte is likewise a simple branching undifferentiated thallus.

(b) The resemblance of the two generations of the life cycle is more striking in this order than in any other group of Pteridophytes.

 The vascular gametophyte is unique in the Plant Kingdom.

(c) The Psilotales have no known fossil history in the geological record. 

Their nearest relatives are a very primitive early Devonian order of fossils, the Psilophytales. 

It is indeed a puzzle as to why the undoubtedly primitive and ancient Psilotales should have survived without leaving us any faint clue in the fossil record as to ‘what they are or whence they came’.

(d) The genera of this order exemplify discontinuous geographical distribution. 

This occurrence pattern at the ends of the earth probably indicates that the Psilotales are merely relics of a once more continuous and widespread group.

(e) In Pteridophytes the gametophyte generation has the haploid or n. number of chromosomes; the sporophyte has the diploid or 2n. number which is halved by a reduction division or meiosis at spore formation.

 The cytology of this order is extremely interesting.

 The chromosome count for the sporophytic cells of the Psilotales is exceedingly high. 

For Tmesipteristhe diploid chromosome number is about 200 (Yeates, 1925), although Manton, 1950, reports a sporophytic number of 400.

 It seems likely therefore that polyploidy, i.e., multiplication of the diploid number, occurs in this genus. 

The fact that there is more than one nucleolus in the nuclei of some specimens, established by Yeates, also supports this.

LATICIFERS Introduction, Structure, Classification and Functions

                            LATICIFERs

INTRODUCTION :-

• Laticifer is a general term applied to a large and heterogenous group of cells that responsible for latex production.
• Found in over 900 genera of plants, laticifers typically function as internal secretory systems.
• 900 genera distributed in 20 families of mostly in dicotyledons and  a few families of monocotyledons
•   Laticifers are highly specialized cells with unique DNA which can produce a wide variety of proteins.
• These proteins include enzymes functioning as proteinases and chitinases which help defend the producing plant against insects and other herbivores

 Defination :-

•       A laticifer is a type of elongated secretory cell found in the leaves or stems of plants that produce latex and rubber as secondary metabolites.
•      The word laticifer and its adjectival form laticiferous are derived from the word latex, meaning juice in Latin.

Latex :-

•     Latex is fluid produced in the latex vessels or cells. It is usually white and milky.
•     Latex is a mixture of many distinct compounds, there is always a predominance of terpenoids in its composition
•      Rubber tree has up to 45% polyisoprenes (rubber) in its latex composition .
•      In addition, fatty acids, phytosterols, alkaloids, phenolic compounds, proteins, cardenolides, starch grains, among other compounds, have already been identified in the latex of many species 

TYPES :-

 Laticifers divided into two type

1- Articulated laticifers

    Composed of a series of cells joined together. Fig (B).

2-Non articulated

It is a very long tube-shaped cell with many nuclei. Fig (A).

Articulated laticifers :-

•       A structure made up of many cells.
•       The laticifers run parallel to the vascular bundles, but just below them.And they are  branched.
•       Laticifers grow through the intercellular spaces and the enzyme pectinase helps in the process.

Non-Articulated :-

•      It is a very long tube-shaped cell with many nuclei .
•      Its walls are scalloped where the surrounding parenchyma cells press slightly into it.
•      The former is derived from the enlargement of a single cell.
•       This cell has the potentiality of unlimited and rapid growth, and elongates to form long latex tubes.
•       The tubes may remain unbranched termed non-articulate unbranched laticifer (e.g. Vinca, Cannabis, Urtica etc.)

Structure of  Laticifers :-

•      Laticifers in some plants contain numerous small vacuoles, or vesicles, which later merge with the central vacuole.
•      Osmiophilic globules are formed in the small vacuoles and later released into the central one.
•      Partial disintegration of protoplasts takes place in the central vacuole, apparently by autophagy.
•       The globules are retained.
•      The laticifers are long thin-walled cells.
•      Many show single perforations in the end walls.

Cell wall :-

•      The cell wall of laticifers is thick and may be thicker than the adjacent cells.
•      They are not lignified.
•      The growth of cell wall occurs through apposition process.
•      The tip of latex cell is thin walled.
•      The walls are composed of cellulose, hemicellulose and pectin.

Function :-

•         It is a vital sap vessel and similar to the blood vessels of animals.
•        It takes part in translocation of assimilates as it is associated with phloem.
•       It stores food materials.
•      It is now considered as secretory tissues where the secretory substances do not re-enter the   plant metabolism.
•      Sen and Chawan (1972) suggested that laticifiers regulate the water balance in plants.
•      It has role in the transport of oxygen.
•      It has role in healing up of wounds.
•      It acts as a defense against herbivores and microorganisms. McCay and  
•      Mahlberg (1973) reported the absence of bacterial activity from laticifers.