SIZE OF PORES AND THEIR CONTENTS IN SIEVE ELEMENTS OF DICOTYLEDONS

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Abstract
The pores and their contents[long dash]that is, the connecting strands and the callose cylindeds incasing the strands[long dash]were measured in 160 species of 120 genera in 60 families of dicotyledons. The diameters of the pores ranged in different species from a fraction of a micron to about 14u. Typically, in the same species, the pores were larger in the sieve areas of sieve plates (end walls bearing comparatively highly diffrentiated sieve areas) than in those in the side walls. The pores and the connecting strands were larger, and the differences in dimensions between the pores and strands of the sieve plates and those in the side walls were greater, in elements with simple sieve plates than in those with compound sieve plates. Sieve plates with scalariform sieve plates have the largest mean transverse cell areas, those with simple sieve plates the smallest. Sample measurements show that the mean areas of the pores per sieve plate are largest in sieve elements with scalariform sieve plates and smallest in those with simple sieve plates. The reverse is true of connecting strands. In the sampling just noted the ratio of pore area to transverse cell area appears to be larger in elements with compound sieve plates than in those with simple sieve plates. The ratio of strand area to transverse cell area, however, showed the reverse: it was largest in elements with simple sieve plates.

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