CLIMBING PLANTS
Humulus lupulus, common hops, is a twining perennial dioecious vine that is grown commercially and prized for the aromatic female flower clusters used to preserve and flavor beer.79 It is also an attractive ornamental that adorns porches and trellises with the aid of myriad tiny hook-like trichomes along the stem that help the vine gain purchase. Of the half dozen Humulus species in the genus, native to Europe, southwestern Asia, and North America, H. lupulus is certainly the most common, with multiple cultivated varieties. It was practically a staple in the England of Darwin’s era; the county of Kent, where Darwin lived, was famous for its hop yards, and he certainly enjoyed a Kentish brew from time to time in his local pubs, the George Inn (now George & Dragon) and Queen’s Head, both still serving today. He would surely have sampled, too, some of the fine brews, including “Natural Selection” and “Darwin’s Origin,” which were produced in 2009 to celebrate the bicentennial of his birth and sesquicentennial of the publication of On the Origin of Species.
Hops was the first plant Darwin wrote about in Movements and Habits of Climbing Plants—a fine choice, as it’s one of the most common representatives of the most common group of climbers he described, “spirally twining plants,” which he defined as “those which twine spirally round a support and are not aided by any other movement.” Humulus is among the small minority of left-handed climbers; viewed from above, they spiral around their support in a clockwise direction, extending from lower-right to upper-left, while the majority of species make right-handed helices, winding counterclockwise.
When the shoot of a Hop (Humulus lupulus) rises from the ground, the two or three first-formed joints or internodes are straight and remain stationary; but the next-formed, whilst very young, may be seen to bend to one side and to travel slowly round towards all points of the compass, moving, like the hands of a watch, with the sun. The movement very soon acquires its full ordinary velocity. From seven observations made during August on shoots proceeding from a plant which had been cut down, and on another plant during April, the average rate during hot weather and during the day is 2 hrs. 8 m. for each revolution; and none of the revolutions varied much from this rate. The revolving movement continues as long as the plant continues to grow; but each separate internode, as it becomes old, ceases to move.
To ascertain more precisely what amount of movement each internode underwent, I kept a potted plant, during the night and day, in a well-warmed room to which I was confined by illness. A long shoot projected beyond the upper end of the supporting stick and was steadily revolving. I then took a longer stick and tied up the shoot, so that only a very young internode, 1¾ of an inch in length, was left free. This was so nearly upright that its revolution could not be easily observed; but it certainly moved, and the side of the internode which was at one time convex became concave, which, as we shall hereafter see, is a sure sign of the revolving movement. I will assume that it made at least one revolution during the first twenty-four hours. Early the next morning, its position was marked, and it made a second revolution in 9 hrs.; during the latter part of this revolution, it moved much quicker, and the third circle was performed in the evening in a little over 3 hrs. As on the succeeding morning, I found that the shoot revolved in 2 hrs. 45 m.; it must have made during the night four revolutions, each at the average rate of a little over 3 hrs. I should add that the temperature of the room varied only a little. … From this time forward, the revolutions were easily observed. The thirty-sixth revolution was performed at the usual rate; so was the last or thirty-seventh, but it was not completed; for the internode suddenly became upright, and after moving to the centre, remained motionless. I tied a weight to its upper end, so as to bow it slightly and thus detect any movement; but there was none. Some time before the last revolution was half performed, the lower part of the internode ceased to move.
A few more remarks will complete all that need be said about this internode. It moved during five days; but the more rapid movements, after the performance of the third revolution, lasted during three days and twenty hours. The regular revolutions, from the ninth to thirty-sixth inclusive, were effected at the average rate of 2 hrs. 31 m.; but the weather was cold, and this affected the temperature of the room, especially during the night, and consequently retarded the rate of movement a little. … After the seventeenth revolution, the internode had grown from 1½ to 6 inches in length, and carried an internode 1⅞ inch long, which was just perceptibly moving; and this carried a very minute ultimate internode. After the twenty-first revolution, the penultimate internode was 2½ inches long, and probably revolved in a period of about three hours. At the twenty-seventh revolution the lower and still moving internode was 8⅜, the penultimate 3½, and the ultimate 2½ inches in length; and the inclination of the whole shoot was such that a circle 19 inches in diameter was swept by it. When the movement ceased, the lower internode was 9 inches, and the penultimate 6 inches in length; so that, from the twenty-seventh to thirty-seventh revolutions inclusive, three internodes were at the same time revolving.
The lower internode, when it ceased revolving, became upright and rigid; but as the whole shoot was left to grow unsupported, it became after a time bent into a nearly horizontal position, the uppermost and growing internodes still revolving at the extremity, but of course no longer round the old central point of the supporting stick. From the changed position of the centre of gravity of the extremity, as it revolved, a slight and slow swaying movement was given to the long horizontally projecting shoot; and this movement I at first thought was a spontaneous one. As the shoot grew, it hung down more and more, whilst the growing and revolving extremity turned itself up more and more.
With the Hop we have seen that three internodes were at the same time revolving; and this was the case with most of the plants observed by me. With all, if in full health, two internodes revolved; so that by the time the lower one ceased to revolve, the one above was in full action, with a terminal internode just commencing to move.
Ipomoea purpurea. Water and bodycolor on vellum by Dame Ann Hamilton, Drawings of Plants.