THE GEOGRAPHICAL DISTRIBUTION OF PRICKLY LETTUCE (LACTUCA SERRIOLA) III. ITS PERFORMANCE IN TRANSPLANT SITES BEYOND ITS DISTRIBUTION LIMIT IN BRITAIN.

(1) The annual ruderal Lactuca serriola, which is confined in Britain to the south-east. was introduced into experimental sites in the north-west (Cheshire) and the south-west (Devon and Cornwall) and its survival and performance at these sites was compared with that within its natural area of distribution. (2) Seeds sown early in the autumn germinated quickly whereas seeds sown later germinated sporadically until December, Germination was affected by soil type, rainfall arid temperature, but was unimpaired beyond its distribution limit. (3) Winter growth was laster in the warmer sites regardless of where they were located relative to its limit. In Cheshire, plants had slightly lower winter dry weights and relative growth rates than in Essex, but the fastest growth was recorded in the south-west. Differences in the pattern of growth between Essex and Cheshire were reflected in higher leaf weight ratios among the Cheshire plants. All these differences disappeared when rapid growth started in the spring. (4) Development towards flowering and fruiting was always laster within its distribution limit than beyond it, but vernalized plants beyond the limit always shed their seeds before the onset of winter. No differences in fecundity within and beyond its limit were detected. (5) in normal years vernalization was essential for the plant to complete its life-cycle, but in the exceptionally hot summer of 1976 some non vernalized plants produced seed within its limit, but none did so beyond it. (6) In controlled environments the rate of development towards flowering was strongly related to temperature. At mean temperatures above 20 °C development was only slightly hastened. whereas below 14 °C it was delayed considerably: between these temperatures the response was linear. (7) Plants regenerated from self-sown seeds and persisted for several generations in all the sites after the experiments were finished. (8) It is concluded that the responses to the climate observed in L. serriola beyond its limit are too small for the limit to he explained in terms of failure in the individual plant. It is suggested that the responses of individuals must he related to their secondary effects on populations and metapopulations if the distribution limit of L. serriola is to be understood. [ABSTRACT FROM AUTHOR]