All their many kinds (and aspens, their kin, must be counted with them) shake themselves perpetually free of the motionless forest. It is easy to gather them. Glances sent into the far distance pay them a flash of recognition of their gentle flashes; and as you journey you are suddenly aware of them close by. Light and the breezes are as quick as the eyes of a poplar-lover to find the willing tree that dances to be seen. No lurking for them, no reluctance. One could never make for oneself an oak day so well. The oaks would wait to be found, and many would be missed from the gathering. But the poplars are alert enough for a traveller by express; they have an alarum aloft, and do not sleep. From within some little grove of other trees a single poplar makes a slight sign; or a long row of poplars suddenly sweep the wind. They are salient everywhere, and full of replies. They are as fresh as streams. It is difficult to realize a drought where there are many poplars. And yet their green is not rich; the coolest have a colour much mingled with a cloud-grey. It does but need fresh and simple eyes to recognize their unfaded life. When the other trees grow dark and keep still, the poplar and the aspen do not darken--or hardly--and the deepest summer will not find a day in which they do not keep awake. No waters are so vigilant, even where a lake is bare to the wind. When Keats said of his Dian that she fastened up her hair "with fingers cool as aspen leaves," he knew the coolest thing in the world. It is a coolness of colour, as well as of a leaf which the breeze takes on both sides--the greenish and the greyish. The poplar green has no glows, no gold; it is an austere colour, as little rich as the colour of willows, and less silvery than theirs. The sun can hardly gild it; but he can shine between. Poplars and aspens let the sun through with the wind. You may have the sky sprinkled through them in high midsummer, when all the woods are close. Sending your fancy poplar-gathering, then, you ensnare wild trees, beating with life. No fisher's net ever took such glancing fishes, nor did the net of a constellation's shape ever enclose more vibrating Pleiades. CLOUD During a part of the year London does not see the clouds. Not to see the clear sky might seem her chief loss, but that is shared by the rest of England, and is, besides, but a slight privation. Not to see the clear sky is, elsewhere, to see the cloud. But not so in London. You may go for a week or two at a time, even though you hold your head up as you walk, and even though you have windows that really open, and yet you shall see no cloud, or but a single edge, the fragment of a form.