Axis elongation can occur with net longitudinal orientation of wall microfibrils

During the initial phases of elongation of pea internodes, oat and rice col eoptiles, oat mesocotyls, soybean hypocotyls and dandelion peduncles, net t ransverse orientation of cellulose wall microfibrils (Mfs) was found in the outer epidermal wall. This paper demonstrates that in all these axes, with the exception of rice coleoptile, net longitudinal orientation of microfib rils occurs in the outer epidermal wall in portions of the axes that were s till elongating at the time of sampling. The timing of the transition to ne t longitudinal orientation and whether the transition proceeded acropetally or basipetally varied with the type of axis under study. The variability o f the relationship between extension and the transition from net transverse to net longitudinal orientation suggests that factors other than extension are important in determining the transition. Layers of longitudinal wall m icrofibrils may be added to the extending epidermal wall to bolster its ten sile strength commensurate with its function during and after extension. At tention is drawn to the parallels between the concept of tissue tension in growing axes and the concept that the epidermis functions as a stressed ski n in the support of mature plant parts in primary growth.