In Nitella, transverse orientation of wall microfibrils (hoop reinforcement) accompanies cell extension, assumedly in a causal fashion. When this model of cell extension is applied to epicotyls and coleoptiles, one must contend with the fact that the outer tangential wall (OTW) of the epidermis, which appears to be the site of rate limiting resistance to elongation has a more or less crossed ply structure. Extension is driven by the development of turgor in the interior tissues, where hoop-reinforcement is the rule. The proposition set forth here is that the crossed-ply wall is an accommodation, during elongation, to the need for increased tensile strength in the OTW because of the relevance of the OTW to organ support. In mature plant parts where elongation is no longer a factor the "tissue tension" aspects of organ extension have a direct counterpart in the "stressed-skin" mechanism for supporting the primary axis under full turgor. Preliminary support for this concept is sought in a comparative study of the OTW in various axes grown under various conditions.

Key words: cell wall, extension growth