| Abstract: | Changes in weight, nitrogen content, and cell fine structurewere followed in ripening cotyledons of greenhouse-grown beans.The seeds mature within 53–56 days from flowering, cotyledonweight and nitrogen content increasing most rapidly betweendays 22 and 34. The cotyledon parenchyma cells first becomevery highly vacuolate, but soon the large vacuoles are dividedup and converted to reserve protein bodies, while cell expansioncontinues. Vacuole subdivision is accompanied by synthesis ofcytoplasm containing masses of rough-surfaced ER (endoplasmicreticulum), which persists till the cotyledons dry out, andpresumably synthesizes the reserve protein. Starch grains growwithin plastids to reach diameters of 50 µ. Young cotyledonsare green but chlorophyll disappears when the seed dries. Mostorganelles are recognizable in dry cotyledon cells; the ER is,however, replaced by small vesicles. Ribosomes are dispersedfree in the cytoplasm during dehydration; this could indicatea destruction of mRNA (messenger ribonucleic acid) in preparationfor a switch to a different metabolic activity during germination. Some comparisons are drawn between cell fine structure in thecotyledons during ripening and germination. |
