This paper describes electron microscopic studies on the distribution of pectin within young plant cells. Dark-grown onion roots, from 1 to 3 mm. in length, were used. In order to make the pectic substances selectively dense to electrons, they were first reacted with basic hydroxylamine. This treatment produces pectic hydroxamic acids, which in turn were treated with ferric ion to form insoluble complexes. The tissue was imbedded, sectioned, and then observed by electron microscopy. Dense deposits of iron were found in the region of the middle lamella and in a second area near the surface of the primary wall. Transverse walls of varying maturity were noted. The pectin of the more frequent, immature cross-walls, leads directly into the inner reacting layer of the axillary (longitudinal) wall. The pectin of the more mature transverse walls becomes, on the other hand, intimately associated with the middle lamella pectin of the axillary wall. It is shown that the pectin of the middle lamella represents the hot water-soluble portion of the pectic substance, while the internal layer of the axillary wall and the transverse wall pectin represent the so called residual fraction. Hot versene extraction removes essentially all electron-dense material.

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