JUDGMENT S. Padmanabhan, J. 1. Second appeal is by the defendant. Suit is one for partition. Respondent plaintiff is his direct brother. They are admittedly joint lessees entitled to the suit property in equal halves. The appellant's contentions are: a) the suit for partition is not maintainable because of an oral partition of 1949, by which he is in separate possession as full owner; b) if not, the title of the respondent in the property in his possession is lost by adverse possession; and c) if the above two contentions are not accepted, he is entitled to value of improvements. 2. Trial court rejected the pleas of oral partition and adverse possession. These findings were confirmed in appeal. Claim for value of improvements was allowed by the trial court. Allowing the cross objection filed by the respondent, the claim for value of improvements was turned down by the appellate court. But a direction was made that, as far as possible, the property in the possession of the appellant will be allotted to his share without valuing the improvements effected by him. Separate possession pleaded by the appellant was accepted by both the courts below. That factual finding must be treated as final and conclusive. 3. The finding against oral partition also is beyond interference. It is not only a conclusion in the absence of sufficient evidence, but the result of a successful plea of res judicata also. O. S. No. 761 of 1966 was filed by the jenmi for realisation of rent of the entire premises. Appellant was the first defendant and respondent was the second defendant. Appellant pleaded oral partition and said that he is liable only for the proportionate rent for the premises in his possession. Respondent denied the oral partition. Both were necessary parties to that suit. Claim of the jenmi was on the original lease to get the entire rent from both the tenants jointly and severally charged 3 on the entire premises. In order to consider and allow or reject that claim, a decision on the conflict between the defendants regarding oral partition was necessary. That was decided, as Exts. A9 and 10 judgment and decree show. The oral partition was found against and the relief claimed by the plaintiff was granted. Claim of the appellant is that Exts. A9 and 10 will not operate as res judicata. 4. That was decided, as Exts. A9 and 10 judgment and decree show. The oral partition was found against and the relief claimed by the plaintiff was granted. Claim of the appellant is that Exts. A9 and 10 will not operate as res judicata. 4. Various decisions, including Sheoparsan Singh and others v. Ramandan Prasad Narayan Singh and others ( AIR 1916 PC 78 ). Mt. Munni v. Tirloki Nath ( AIR 1931 PC 114 ), Kishun Prasad Pandey and others v. Durga Prasad Thakur and others ( AIR 1931 PC 231 ), Maung Sein Done v. Ma Pan Nyun ( AIR 1932 PC 161 ), Kedar Nath v. Munshi Ram ( AIR 1935 PC 139 ). Syed Mohammad Saadat Ali Khan v. Mirza Wiquar Ali Beg and others ( AIR 1943 PC 115 ), Chandu Lal Aarwalla and another v. Khalilur Rahaman and others ( AIR 1950 PC 17 ), Shashibhushan Prasad Mishra and another v. B Babuji Rai and others ( AIR 1970 SC 809 ), Iftikhar Ahmed and others v. Syed Meharban Ali and others ( AIR 1974 SC 749 ) and Janardhanan Pillai v. Kochunarayani Amma (1976 KLT 279), considered the question of res judicata between codefendants. The rule of res judicata while founded on ancient precedent is dictated by a wisdom for all time and the application of the rule should be influenced by no technical considerations of form, but by matter of substance within the limits allowed by law. The raison de'tre (reason or purpose) of the rule is to confer finality on decisions arrived at by competent courts between interested parties after genuine contest. It corresponds to what is known as estoppel by judgment in English law. It is based partly on the rule of public policy that it concerns the State that there be an end of law suits and partly on private justice that no man should be vexed twice over for the same cause. S.11 of the Code of Civil Procedure grants a prohibitory injunction against courts not to try a suit or issue in which the matter was directly and substantially in issue between the parties or under whom they claim and heard and finally decided provided some other conditions are also satisfied. S.11 of the Code of Civil Procedure grants a prohibitory injunction against courts not to try a suit or issue in which the matter was directly and substantially in issue between the parties or under whom they claim and heard and finally decided provided some other conditions are also satisfied. Directly and substantially in issue could only be if it was alleged and denied and was a matter of importance and value in the litigation and necessary for its disposal. It is in contradistinction with matters collaterally and incidentally in issue. Alleged and denied could normally be only between parties arrayed on opposite sides, as plaintiff and defendant. 5. But, in certain cases, it could as well be between parties arrayed on the same side, as plaintiffs or defendants, though it is rare. Res judicata as between co-plaintiffs may be rarer than that between codefendants. But the same principles may apply in both cases. In both cases, the rule will have to be applied with great caution and only if the following conditions are satisfied. There must be conflict of interest between them. They must be necessary or proper parties. It was necessary to decide that conflict in order to give the relief, which the plaintiff claimed. The court must have actually decided that conflict finally on the merits. The doctrine may apply even though a party, against whom it is sought to be enforced, did not think fit in the previous suit to enter appearance and contest the question provided that he had or must be deemed to have had notice that the relevant question was in issue and would have to be decided. That is in the principle of constructive res judicata based on the might and ought theory. If all these conditions are satisfied, there can be res judicata not only between codefendants, but between co-plaintiffs so. All these conditions are satisfied in the present suit and hence Exts. A9 and A10 must operate as res judicata against the appellant in his claim of oral partition. 6. The plea of adverse possession is also equally untenable in spite of proof of separate possession and separate payment of rent. Separate claim for tree cutting compensation or separate purchase certificates also cannot help when the oral partition is found against and when the parties are found to continue as coowners. 6. The plea of adverse possession is also equally untenable in spite of proof of separate possession and separate payment of rent. Separate claim for tree cutting compensation or separate purchase certificates also cannot help when the oral partition is found against and when the parties are found to continue as coowners. It is true that in 1964 the respondent issued a notice for partition and the appellant replied by saying that they are divided. But O. S. No. 761 of 1966 was instituted and the decision rendered subsequently. Separate lease deed was executed in favour of the jenmi only by the appellant and not the respondent. 7. In order to succeed in a plea of adverse possession, the possession required must be adequate in continuity, in publicity and in extent and it must be with the requisite hostile animus to the knowledge of the opposite party. As held in Lakshmi Reddy v. Lakshmi Reddy ( AIR 1957 SC 314 ) and various other decisions, the law relating to adverse possession, as between coowners, is now well settled and it is more rigorous than in the case of others. That is because one coowner is rather in the position of a fiduciary capacity akin to that of a trustee, so far as other coowners are concerned. Exclusive possession of the claimant or absence of receipt of rents and profits even for a long period by itself may not be sufficient. That is because until partition, the coowner is in possession will be deemed in law to possess on behalf of the other coowners also. Relinquishment of the fiduciary relationship by ouster of the non possessing coowner by the coowner in possession, who claims adverse possession, must be made out for the statutory period. When one coowner is found to be in possession, the presumption is that it is on the basis of joint title. The coowner in possession cannot render his possession adverse to the other coowners not in possession merely by any secret hostile animus on his own part in derogation of the other coowners' title. There must be evidence of open assertion of hostile title occupied with exclusive possession and enjoyment by one of them to the knowledge of others so as to constitute an act of ouster to their knowledge. There must be evidence of open assertion of hostile title occupied with exclusive possession and enjoyment by one of them to the knowledge of others so as to constitute an act of ouster to their knowledge. In the absence of these factors, separate possession, payment of tax or rent, separate claims for compensation and separate purchase certificates could be deemed to be only on behalf of the other coowners also. Appellant has not claimed or proved any ouster or possession with hostile animus. He claimed only separate possession on the basis of an oral arrangement, which is denied and found against. The plea of adverse possession was also rightly found against. 8. There is nothing to show that separate funds, other than the income of the property, were spent for effecting improvements. A coowner in possession cannot be treated as a tenant coming under any of the three categories enumerated in S.2 (d) of the Kerala Compensation for Tenants Improvements Act. He is not a lessee, sublessee, mortgagee sub mortgagee or a person in good faith believing to be so. He is also not a person bona fide attorning or a person coming into possession of the land of another and makes improvements in the bona fide belief that he is B entitled to make such improvement. He is also not a person coming within the meaning of S.51 of the Transfer of Property Act. He is an owner of every inch of the coownership land until partition. He cannot improve a coowner out of the estate by claiming value. Unless authorised by the other coowners and he effected improvements with his own funds, he cannot claim value from them. The maximum that he could request for, at any rate, is an equity that to the extent possible, the area improved may be allotted to his share without valuing the improvements effected by him. That was granted by the appellate court and its correctness is not challenged. Hence I need not consider the correctness of that direction. There was no basis for him to come up in second appeal. Second appeal is dismissed, with costs.