PHYSIOLOGICAL‐ECOLOGY OF XANTHIUM STRUMARIUM LINN.

S ummary There are various forms within the species which exhibit a differential seasonal behaviour. These have been designated as ‘monsoon’, ‘winter’, ‘winter‐summer’ and ‘summer’ forms. The life cycle of the ‘monsoon form’ starts under a high temperature regime and finishes under low temperature. The ‘winter’, ‘winter‐summer’, and the ‘summer’ forms grow in constant low temperature, an initial low temperature terminating in a high temperature and in constantly‐high temperature regimes respectively. The critical photoperiod for these forms ranges from 13 hours' duration to continuous light. The species does not grow at all in shaded localities. Many of the differentiating characters, however, become slightly less distinct when the different forms are raised together under a similar set of conditions in one or the other season, and this suggests that they originated from a common stock and evolved in response to environmental differences (as has been suggested for the various American species of Xanthium ). The origin of the species with regard to its varying critical photoperiod has been discussed.