The poplar.—The various poplars are also completely unisexual, that is, any one tree bears either male or female flowers, but not both. In this case also the flowers appear before the leaves; but the flowers are pollinated by the aid of the wind, not by insects, and nectaries are therefore not formed. The catkins are not very much like those of the willows in appearance, but the fruit and seeds are very similar. Both willows and poplars are fond of the banks of streams. The English poplars are graceful trees which can be recognised at a distance by the manner in which the broad leaves turn on their long stalks, exposing the lighter-coloured under-surface at the slightest breath of wind. This is especially noticeable with the aspen or trembling poplar ([Fig. 122]). The Lombardy poplar is a tall, stiff tree.
Fig. 122.—The Poplar. 1, male catkin (× ⅔); 2, female catkin (× ⅔); 3, male flower (× 2); 4, female flower (× 4); 5, the same in longitudinal section; 6, fruit; 7, the same after opening; 8, seeds (× 3); 9, diagram of male flower.
32. THE ELM, LIME, AND ASH.
1. The elm.—Notice the straight trunk, rough bark, and slender branches of the common elm. Examine the flowers in early spring and notice that each contains both stamens and pistil. When do the flowers open? When do the leaves expand? Draw a leaf and a fruit. What is the use of the broad plate of the fruit?
2. The lime.—Observe the straight trunk, smooth bark, and general pyramid shape of the tree. Draw a leaf, and notice that it is pointed and is larger on one side of the midrib than on the other. Notice that the flower stalks spring from leaves (bracts) differing in shape ([Fig. 126]) and colour from ordinary leaves. Draw one of these leaves with its attached flower stalk. When do the flowers open? Try to discover whether bees haunt the flowers. Find out what is the use to the fruit of the long leafy bract.
3. The ash.—Look in winter for the black buds and the flattened tips of the twigs. When do the flowers open? Notice their rich purple colour. Examine a flower and make out the two stamens and the pistil. Trace the formation of the fruit, which is winged and hangs in bunches called keys. What is the use of the wing? When do the leaves expand? Are they simple or compound? Draw one. What is its colour? When do the leaves fall?
