THE DEVELOPMENT OF DORMANCY IN SEEDS OF COCKLEBUR (XANTHIUM)

Abstract

In a previous paper (2) it was shown that dormancy may be induced in the after-ripened embryos of ragweed (Ambrosia trifida L.) by means of high temperature germination in connection with restricted oxygen supply to the embryos due to the fruit and seed membranes that envelop them. These embryos, however, were dormant at maturity and the induced secondary dormancy was evidently merely a reversal of the essential changes through which the embryos had gone in the after-ripening process or the removal of the original or primary dormancy. The embryos of seeds of Xanlhium canadense and X. commune have at no time during periods of dry storage of seeds in the burs exhibited any tendency to dormancy when placed under germinating conditions. No doubt this is also true of other species of Xanthium. Shull (4) compared the germination of embryos of seeds of X. glabratum, when quite green, with those of fully ripened seeds and of seeds one year old and was able to detect no perceptible after-ripening in passing from the unripe to the ripe and year-old conditions. The so-called dormancy or delay in the germination of these seeds at certain temperatures is due, as pointed out first by Crocker (i), to the restriction of the oxygen supply to the embryos by the seed coats for when the seed coats are removed and the naked embryos are placed under suitable conditions, germination usually takes place within 24 to 48 hours.