Histology and histochemistry of the cotyledons of pisum arvense L. during germination.

The mature cotyledon of Pisum arvense L. comprises several distinct tissue regions; these are the epidermis, hypodermis, storage parenchyma and procambium. The storage parenchyma includes two zones: an outer abaxial zone and an inner adaxial zone. The cells of both zones contain abundant starch grains and protein bodies. Scattered through the storage tissue but increasing in frequency towards the periphery are certain cells which differ to a slight extent from the majority of the parenchyma cells. They have a more opaque, granular cytoplasm and a higher level of cytoplasmic RNA. the cotyledon has a complex, reticulate vascular system. Differentiation of the conducting elements from the procambium appears to begin about 12 hours and to be completed 48 hours after the commencement of imbibition. Differentiation of phloem preceeds that of xylem. The relationship between the timing of vascular differentiation and various physiological events in the cotyledon is discussed.Mobilization of the reserves in the storage parenchyma is initiated at the periphery of the cotyledon and proceeds inwards. There appears to be a correlation between the breakdown of the reserves and changes in DNA and RNA content of the cells.