Application of a Thermal Gradient Bar to the Study of Germination Patterns in Successional Herbs

A thermal gradient bar was used to investigate the germination temperature optima (GTO) for various successional herbs. Four species of Plantago were investigated: P. major and P. rugelii had a GTO of about 28 C, whereas P. aristata and P. virginica showed a GTO of about 16 C. These optima were correlated with germination patterns in the field. The first pair normally germinates in early summer at relatively high mean surface soil temperatures; the second pair germinates in fall at low mean surface soil temperatures. The thermal gradient bar appears to be a useful tool in dealing with these relations of phenology and physiology. INTRODUCTION Few events are as critical in determining successional pattern as seed germination and the environmental factors that control it. The present study was designed to discover whether germination temperature optima determined in the laboratory bore any relation to germination patterns observed in the field. In the laboratory, seeds must be germinated in sufficient numbers to employ statistical methods; there must be a wide range of constant temperatures in graded intervals of a few degrees; and finally, high humidity must be maintained. Incubation and environmental chambers can provide these characteristics, but to encompass a broad spectrum of constant temperatures in this way would require a replication of facilities that is rarely available. A thermal gradient device meets many of these problems successfully. The thermal gradient bar consists of a bar or plate of copper, aluminum or stainless steel which is heated at one end and cooled at the other, thereby establishing a thermal gradient. Herter (1924), Nieschulz (1933) and Coulianos (1955) have used such instruments to determine preferential temperature optima for insects and Herter (1934) has used a similar device for mammafs. Some work has also been done to determine the temperature optima for growth of microorganisms in culture by placing the nutrient medium directly on the bar (Landman et al., 1962; Elliott, 1963; Fries and Kaillstromer, 1965), in tubes laid on the bar (Cannefax, 1962), or in tubes inserted into the bar itself (Battley, 1965). Acknowledgments.-The author wishes to acknowledge the skilled help of the members of the various shops at Brookhaven National Laboratory where the bar was constructed and the technical assistance of Mr. Gerard Courtin. Research was carried out at Brookhaven National Laboratory under the auspices of the U. S. Atomic Energy Commission. 1 Present address: Botany Department, Pennsylvania State University, University Park 16802.