The changes in levels of metabolites during photoinduced germination of Onoclea sensibilis L. spores are described.Proteins and lipids, which constitute 25 and 20%, respectively, of the unimbibed spores on a dry weight basis, are hydrolyzed at the time of differentiation and elongation of the germling cells and may be utilized for these processes.Sucrose degradation, starch synthesis, and active respiration occur during dark imbibition, but these processes are accelerated by red or far red irradiation. Endogenous sucrose is the probable source of the carbon skeleton for starch synthesis.An imbibed fern spore given the proper inductive stimulus is able to germinate in the absence of exogenous nutrients (10 (1.1 X 10' ergs cm-2 sec-1; 50% transmittance at 720 nm) and then placed back in the dark for varying periods of time. The spores were then collected and lyophilized as described previously (27). Controls were incubated in the dark throughout the experimental period. Germination was scored by either the acetocarmine procedure or the protrusion method (25). Experiments were repeated at least twice with essentially the same results.The broad band red and far red filters and the dim green safelight were identical to those described previously (25).Extraction Procedure for Soluble Carbohydrates, Proteins, Amino Acids, and ATP. Fifty milligrams oflyophilized spores were extracted by grinding with sand alone and then with 1.5 ml of ice-cold 5% (v/v) perchloric acid for 10 min in a mortar with a pestle, and the brei was centrifuged at 20,000g for 10 min. The pellet was re-extracted with 1 ml of 5 % PCA2 and centrifuged as above. This re-extraction procedure was necessary to extract fully the metabolites. The pellet from the second centrifugation was used for protein determination. The perchlorate was precipitated from the combined supernatants by the addition of sufficient 1 M K2CO4 to raise the pH to 2 to 3. Aliquots of the supernatant were then used for determination of total soluble carbohydrates, amino acids, and ATP. The concentration of the metabolites was calculated on the basis of 1 mg of lyophilized spores.Protein Determination. The PCA-insoluble pellet was solubilized by heating at 75 C for 4 hr with 0