1. Species of Xanthium found commonly on the shores of the St. Lawrence River at Montreal are X. italicum Mor., X. pennsylvanicum Wallr., X. inflexum Mack. & Bush, and a form thought to be X. orientale Linn., but of the described American species most like X. curvescens Millsp. & Sherff. 2. These species all blossom at about the same time, and experiment has shown that hybridization is possible between them. 3. Plants of X. italicum with some burrs of the X. Wootoni type have been found wild here. They fit the description of the "duplex specimen" found by Cockerell in New Mexico. 4. Plants of all the species studied and some hybrid forms, when forced in the greenhouse, produced some X. Wootoni burrs. 5. In hybrids, the characters of X. italicum apparently are dominant to those of any of the other three species grown here. 6. In the species and hybrids thus far investigated, the reduced chromosome number was found to be 18. 7. In garden observations upon delayed germination during two summers, it has been ...