The indumentum of P. alba, P. tremula and their hybrid (P. ×canescens) is discussed in detail and illustrated by scanning electron microscope and light microscope micrographs. Special attention is given to the hairiness of stems and leaves and to the diagnostic value of hairiness in distinguishing the grey poplar from its parent species. Presence or lack of hairs on leaves and stems, their density and their persistence are very useful features in distinguishing the taxa. Short shoot leaves of P. alba are usually described as initially densely white tomentose, but become glabrous or glabrescent. From our observations, it appears that the abaxial leaf surface of the white poplar remains hairy during the whole life span of the leaf. Hairiness does not disappear, but changes with time; arachnoid hairs become squeezed and pressed together until they finally form a very thin shiny layer on the leaf surface. In late summer, the indumentum becomes thinner and leaf blades seem to be glabrous, although hairs are still visible when using a strong magnifying glass. This feature allows easy distinction between P. alba and the similar P. ×canescens, the leaves of which are loosely arachnoid-tomentose when young and become fully glabrous or subglabrous when mature. Due to the presence of arachnoid hairs, the grey poplar can usually also be distinguished from its second parent, P. tremula. However, because of the introgression between the poplars, all possible features should be taken into account in diagnosis, especially the shape and indumentum of terminal adult leaves of long shoots and/or stronger suckers. In P. tremula, such leaves are usually broadly ovate, cordate at the base, irregularly serrate, glabrous or hairy, but never with long arachnoid hairs. In P. alba, the upper leaves of long shoots and suckers are deeply lobed, serrate and densely white arachnoid on the abaxial side, while in P. ×canescens they are serrate or shallowly lobed, rather loosely covered with arachnoid hairs and greenish-grey.
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