Polarized Light Study of Phloem Differentiation in Embryo of Chenopodium album

In both cleared material and paraffin sections differentiating sieve elements may be detected by the double refraction of their nacreous walls as seen in polarized light. Differentiation begins in the lower part of the cotyledons and proceeds basipetally into the hypocotyl and acropetally in the cotyledons. In the mature embryo the differentiating protophloem appears in the form of networks in the cotyledons; thus the cotyledonary venation system is initiated during embryogeny. The protophloem of the cotyledons is continuous with that of the hypocotyl, and differentiating sieve elements are found approximately 100from the root apex. Electron microscopic observation revealed that sieve elements of the mature embryo are only partly differentiated: they still have nuclei. Differentiation of the secondary walls in protoxylem does not occur until after germination, and their appearance in polarized light is distinct from that of the protophloem.