Observations on the Fine Structure of Plant Cell Walls III. The Sieve Tube Wall in Cucurbita

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Abstract
The sieve tube wall in Cucurbita was examined in ultra-thin sections of petioles treated in different ways for the removal of non-cellulosic wall components. The sections were stained with permanganate. The microfibrillar components of the wall are arranged in concentric lamellae. The earliest (outermost) part of the wall is similar to that of ordinary parenchyma in having its lamellae composed of thinly-distributed microfibrils readily separated from one another by certain treatments such as pectinase extraction. In the characteristically-thickened inner (nacreous) layer the microfibrils are very densely packed and the lamellae do not separate readily. The microfibrils in this layer of the wall are very close to transverse and the ‘crossed fibrillar’ orientation is not easily discernible.