This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
The so-called Populus nigra has recently been found to have been generally confounded with the Canadian 31ack Poplar, which is extensively planted in this country (Populus monilifera). The latter has the leaves broadly triangular, and greyish-green. The trunk is much smoother, the branches curve upwards. The leaf-buds are sticky. The catkins are loose. In the true P. nigra the leaves are triangular with a long point, smooth below, and yellowish-green. The young buds are sticky. The branches spread horizontally. The trunk has rough bosses. The style is 2-lobed at the apex. The catkins are loose. The habitat is hedgerows. The tree grows to a height of 60-90 ft., and flowers in April and May.