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A chair includes a supporting frame and a seat portion which is foldable about a transverse fold to define a rearward portion behind the transverse fold and a forward portion, forward of the transverse fold. The seat portion is supported above the supporting frame by its rearward portion. The chai

A chair includes a supporting frame and a seat portion which is foldable about a transverse fold to define a rearward portion behind the transverse fold and a forward portion, forward of the transverse fold. The seat portion is supported above the supporting frame by its rearward portion. The chair also includes a reclinable back portion and a recline mechanism with which the back portion is connected for reclining action of the back portion. The recline mechanism is operably linked to the rearward portion of the seat portion such that on reclining action of the back portion, the rearward portion is moved to increase in rearward tilt angle and to obtain a net increase in height above the supporting frame, with a consequent folding of the seat portion about the transverse fold line under the weight of the occupant.

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What is claimed is: 1. A chair comprising: a supporting frame; a seat portion which is foldable about a transverse fold to define a rearward portion behind the transverse fold and a forward portion forward of the transverse fold, the seat portion being supported above the supporting frame by its re

What is claimed is: 1. A chair comprising: a supporting frame; a seat portion which is foldable about a transverse fold to define a rearward portion behind the transverse fold and a forward portion forward of the transverse fold, the seat portion being supported above the supporting frame by its rearward portion; a reclinable back portion; and a recline mechanism with which the back portion is connected for reclining action of the back portion, the recline mechanism being operably linked to the rearward portion of the seat portion such that on reclining action of the back portion, the rearward portion is moved to obtain a net increase in height above the supporting frame and to increase in rearward tilt angle so that a first portion of the rearward portion that is closest to the fold has a greater net increase in height than a second portion of the rearward portion that is farthest from the fold, the seat portion also being configured so that a consequent downward folding of the forward portion about the transverse fold is caused by weight of an occupant's legs when the occupant is seated on the seat portion. 2. The chair as claimed in claim 1 wherein the seat portion is constructed of a resiliently flexible material. 3. The chair as claimed in claim 2 wherein the seat portion comprises a panel which has apertures to enhance its flexibility. 4. The chair as claimed in claim 1 wherein the seat portion takes the form of a panel and stiffening webs are provided which offer resistance to folding towards the rear of the seat portion and lesser amount of resistance to flexing towards the forward edge of the seat portion. 5. The chair as claimed in claim 4 wherein the stiffening webs are tapered to offer the progressively increasing resistance to folding from the front edge of the seat portion towards the rear. 6. The chair as claimed in claim 1 wherein the rearward portion of the seat portion is supported, at least in part, by the recline mechanism and the forward portion is substantially unsupported. 7. A chair comprising: a supporting frame; a seat portion which is foldable about a transverse fold to define a rearward portion behind the transverse fold and a forward portion, forward of the transverse fold, the seat portion being supported above the supporting frame by its rearward portion; a reclinable back portion; and a recline mechanism with which the back portion is connected for reclining action of the back portion, the recline mechanism being operably linked to the rearward portion of the seat portion such that on reclining action of the back portion, the rearward portion is moved to obtain a net increase in height above the supporting frame, with a consequent folding of the seat portion about the transverse fold under the weight of the occupant, the recline mechanism comprising a four bar linkage comprising: a main support forming part of the supporting frame; a second linkage comprising the seat portion or a guide relative to which the seat portion is selectively movable; a front support linkage extending between the main support and the second linkage; and a drive linkage wherein the drive linkage is pivotable about a drive axis through the main support, the drive linkage being connected to the guide and being operably linked to be driven about the drive axis by rearward recline action of the back portion to bring about the net increase in height of the second linkage on rearward recline action of the back. 8. The chair as claimed in claim 7 wherein two such four bar linkages are defined on opposite sides of the chair. 9. The chair as claimed in claim 8 wherein the main support is selectively height adjustable. 10. A chair comprising: a supporting frame; a seat portion which is foldable about a transverse fold to define a rearward portion behind the transverse fold and a forward portion, forward of the transverse fold, the seat portion being supported above the supporting frame by its rearward portion, the seat portion being adjustable in position between a forward extended position and a rearward retracted position such that the positioning of the transverse fold is variable as a function of the seat depth position; a reclinable back portion; and a recline mechanism with which the back portion is connected for reclining action of the back portion, the recline mechanism being operably linked to the rearward portion of the seat portion such that on reclining action of the back portion, the rearward portion is moved to obtain a net increase in height above the supporting frame, with a consequent folding of the seat portion about the transverse fold under the weight of the occupant. 11. The chair as claimed in claim 10 wherein the recline mechanism includes at least one guide and the seat portion is slidable relative to the guide between the extended and retracted positions, the guide including a fixed portion about which the seat portion folds. 12. The chair as claimed in claim 11 wherein the guide has an upper surface having a forward portion with an upwardly facing convex shape whereby a transition in curvature defines the transverse fold position of the seat portion. 13. The chair as claimed in claim 11 wherein the recline mechanism comprises a four bar linkage comprising four elements as follows: a main support forming part of the supporting frame; the guide; a front support linkage extending between the main support and the guide; and a drive linkage wherein the drive linkage is pivotable about a drive axis through the main support, the drive linkage being connected to the guide and being operably linked to be driven about the drive axis by rearward recline action of the back portion to bring about the net increase in height of the guide on rearward recline action of the back. 14. The chair as claimed in claim 13 wherein two such four bar linkages are defined on opposite sides of the chair. 15. The chair as claimed in claim 14 wherein the back portion is pivotally connected to the main support at a recline axis. 16. The chair as claimed in claim 15 wherein the recline axis is located below the seat portion. 17. The chair as claimed in claim 16 wherein the recline axis is located below the ischial protuberosities of the chair occupant. 18. The chair as claimed in claim 13 wherein the back portion is biased against reclining by a recline biasing device. 19. The chair as claimed in claim 18 wherein the recline biasing device comprises one or more springs. 20. The chair as claimed in claim 19 wherein two back extension arms substantially rigidly extend from the back portion and pivotally connect the back portion to the main support, the one or more springs being held by one or both of the back extension arms, with the spring(s) acting against the main support. 21. The chair as claimed in claim 20 wherein the biasing force of the recline biasing device is adjustable. 22. The chair as claimed in claim 21 wherein two springs are provided, being a first spring and a second spring, both of which operate in the manner of leaf springs with the second spring being clampable against the first spring with the combination having a resultant spring rate, with the degree of clamping being variable to adjust the resultant spring rate. 23. A chair comprising: a supporting frame; a seat portion which is foldable about a transverse fold to define a rearward portion behind the transverse fold and a forward portion forward of the transverse fold, the seat portion being supported above the supporting frame by its rearward portion; a reclinable back portion; and a recline mechanism with which the back portion is connected for reclining action of the back portion, the recline mechanism being operably linked to the rearward portion of the seat portion such that on reclining action of the back portion, the rearward portion is moved to obtain a net increase in height above the supporting frame, the seat portion also being configured so that a consequent downward folding of the forward portion about the transverse fold is caused by weight of an occupant's legs when the occupant is seated on the seat portion, wherein the recline mechanism is operably linked to the rearward portion of the seat portion such that on reclining action of the back portion, the entire seat portion is moved in a horizontal direction relative to the supporting frame. 24. The chair as claimed in claim 23 wherein the back portion comprises a flexible frame which is flexible or at least flexible at a part corresponding to the lumbar region of the occupant. 25. The chair as claimed in claim 24 wherein the flexibility of the back portion is adjustable. 26. The chair as claimed in claim 25 wherein the flexibility adjustment takes place automatically in response to the weight imparted by the occupant on the seat portion with, at least beyond a predetermined threshold in weight, the larger the weight, the greater the stiffness imparted to the back portion. 27. The chair as claimed in claim 26 wherein the flexibility adjustment takes place through the use of a tensionable biasing device comprising at least one leaf-type spring lying against the back portion at a lower region thereof. 28. The chair as claimed in claim 27 wherein the recline mechanism comprises a four bar linkage comprising four elements as follows: a main support forming part of the supporting frame; a second linkage comprising the seat portion or a guide relative to which the seat portion is selectively movable; a front support linkage extending between the main support and the second linkage; and a drive linkage wherein the drive linkage is pivotable about a drive axis through the main support, the drive linkage being connected to the second linkage and being operably linked to be driven about the drive axis by rearward recline action of the back portion to bring about a net increase in height of the second linkage on rearward recline action of the back; and further wherein an interconnecting linkage interconnects the leaf-type spring with the drive linkage such that, at least beyond the predetermined threshold, the weight on the seat causes the leaf-type spring to flex against the back portion to impart greater stiffness thereto. 29. The chair as claimed in claim 28 wherein a supplementary spring is provided, whereby weight on the seat portion up to the predetermined threshold causes flexing of the supplementary spring. 30. The chair as claimed in claim 28 wherein two four bar linkages are defined on opposite sides of the chair, there being two such leaf-type springs on opposite sides of the chair with two such interconnecting linkages, wherein each interconnecting linkage is disposed directly between the associated leaf-type spring and the associated drive link. 31. The chair as claimed in claim 28 wherein a tension limit is provided to prevent over-tensioning of the tensionable biasing device. 32. The chair as claimed in claim 23, wherein at least part of the forward portion of the seat portion is flexible. 33. The chair as claimed in claim 23, wherein the stiffness of the forward portion of the seat portion differs from the stiffness of the rear portion of the seat portion. 34. The chair as claimed in claim 23, wherein the forward portion of the seat portion comprises a downwardly directed forward edge. 35. A chair comprising: a supporting frame; a seat portion which is foldable about a transverse fold to define a rearward portion behind the transverse fold and a forward portion forward of the transverse fold, the seat portion being supported above the supporting frame by its rearward portion; a reclinable back portion; and a recline mechanism with which the back portion is connected for reclining action of the back portion, the recline mechanism being operably linked to the rearward portion of the seat portion such that on reclining action of the back portion, the rearward portion is moved to obtain a net increase in height above the supporting frame, the seat portion also being configured so that a consequent downward folding of the forward portion about the transverse fold is caused by weight of an occupant's legs when the occupant is seated on the seat portion, wherein the seat portion comprises opposing side portions and a central portion disposed therebetween, the seat portion being supported by the recline mechanism at the opposing side portions so that the central portion of the seat portion is freely suspended and unsupported. 36. The chair as claimed in claim 35, wherein the recline mechanism comprises: a pair of spaced apart rails operably connected to the opposing side portions of the seat portion; and a pair of links, each link extending from a corresponding rail to the supporting frame, each link being rotatably mounted to the supporting frame and rotatably mounted to the corresponding rail. 37. The chair as claimed in claim 36, further comprising a seat depth adjustment mechanism securing the rails to the seat portion, the seat depth adjustment mechanism enabling selective positioning and locking of the seat portion along the rails. 38. A chair comprising: a supporting frame; a seat portion which is foldable about a transverse fold to define a rearward portion behind the transverse fold and a forward portion forward of the transverse fold, the seat portion being supported above the supporting frame by its rearward portion; a reclinable back portion; and a recline mechanism with which the back portion is connected for reclining action of the back portion, the recline mechanism being operably linked to the rearward portion of the seat portion such that on reclining action of the back portion, the rearward portion is moved to obtain a net increase in height above the supporting frame, the seat portion also being configured so that a consequent downward folding of the forward portion about the transverse fold is caused by weight of an occupant's legs when the occupant is seated on the seat portion, wherein the recline mechanism comprises: a pair of rails operably connected to the seat portion; and a pair of links, each link extending from a corresponding rail to the supporting frame, each link being rotatably mounted to the supporting frame and rotatably mounted to the corresponding link such that when the back portion is reclined, the pair of links facilitates movement of the entire seat portion in a horizontal direction relative to the supporting frame. 39. A chair comprising: a supporting frame; a seat portion which is foldable about a transverse fold to define a rearward portion behind the transverse fold and a forward portion forward of the transverse fold, the seat portion being supported above the supporting frame by its rearward portion; a reclinable back portion; and a recline mechanism with which the back portion is connected for reclining action of the back portion, the recline mechanism being operably linked to the rearward portion of the seat portion such that on reclining action of the back portion, the rearward portion is moved to obtain a net increase in height above the supporting frame, the seat portion also being configured so that a consequent downward folding of the forward portion about the transverse fold is caused by weight of an occupant's legs when the occupant is seated on the seat portion, wherein the seat portion comprises a transition zone having a plurality of transverse fold positions about which the front portion of the seat portion can downwardly fold under the weight of an occupant's legs when the occupant is seated on the seat portion during a reclining action of the back portion. 40. The chair as claimed in claim 23, wherein the forward portion of the seat portion is cantilevered from the rearward portion of the seat portion by the transverse fold such that the forward portion extends horizontally forward of the supporting frame so as to be unsupported from below. 41. A chair comprising: a supporting frame; a seat portion which is foldable about a transverse fold to define a rearward portion behind the transverse fold and a forward portion forward of the transverse fold, the seat portion being supported above the supporting frame by its rearward portion; a reclinable back portion; and a recline mechanism with which the back portion is connected for reclining action of the back portion, the recline mechanism being operably linked to the rearward portion of the seat portion such that on reclining action of the back portion, the rearward portion is moved to obtain a net increase in height above the supporting frame, the seat portion also being configured so that a consequent downward folding of the forward portion about the transverse fold is caused by weight of an occupant's legs when the occupant is seated on the seat portion, wherein the rearward portion of the seat portion is operatively supported by a seat depth adjustment mechanism such that the seat portion is adjustable relative to the supporting frame between a forward extended position and a rearward retracted position. 42. A chair comprising: a supporting frame; a seat portion which is foldable about a transverse fold to define a rearward portion behind the transverse fold and a forward portion forward of the transverse fold, the seat portion being supported above the supporting frame by its rearward portion; a reclinable back portion; and a recline mechanism with which the back portion is connected for reclining action of the back portion, the recline mechanism being operably linked to the rearward portion of the seat portion such that on reclining action of the back portion, the rearward portion is moved to obtain a net increase in height above the supporting frame, the seat portion also being configured so that a consequent downward folding of the forward portion about the transverse fold is caused by weight of an occupant's legs when the occupant is seated on the seat portion, wherein the recline mechanism comprises a plurality of supports that support the seat portion from the supporting frame such that on reclining action of the back portion, movement of the seat portion relative to the supporting frame occurs, wherein said movement of said seat portion comprises a horizontal component of movement and further comprises a vertical component of movement, wherein the vertical component of movement comprises said net increase in height of the rearward portion of the seat portion above the supporting frame.