第22章 LEAF-CLIMBERS(10)

The young leaves, when first developed, stand up nearly vertically;but by the growth of the axis, and by the spontaneous bending down of the terminal half of the leaf, they soon become much inclined, and ultimately horizontal.The end of the leaf forms a narrow, ribbon-like, thickened projection, which at first is nearly straight, but by the time the leaf gets into an inclined position, the end bends downwards into a well-formed hook.This hook is now strong and rigid enough to catch any object, and, when caught, to anchor the plant and stop the revolving movement.Its inner surface is sensitive, but not in nearly so high a degree as that of the many before-described petioles; for a loop of string, weighing 1.64 grain, produced no effect.When the hook has caught a thin twig or even a rigid fibre, the point may be perceived in from 1 hr.to 3 hrs.to have curled a little inwards; and, under favourable circumstances, it curls round and permanently seizes an object in from 8 hrs.to 10 hrs.The hook when first formed, before the leaf has bent downwards, is but little sensitive.If it catches hold of nothing, it remains open and sensitive for a long time; ultimately the extremity spontaneously and slowly curls inwards, and makes a button-like, flat, spiral coil at the end of the leaf.One leaf was watched, and the hook remained open for thirty-three days; but during the last week the tip had curled so much inwards that only a very thin twig could have been inserted within it.As soon as the tip has curled so much inwards that the hook is converted into a ring, its sensibility is lost; but as long as it remains open some sensibility is retained.

Whilst the plant was only about six inches in height, the leaves, four or five in number, were broader than those subsequently produced; their soft and but little-attenuated tips were not sensitive, and did not form hooks; nor did the stem then revolve.At this early period of growth, the plant can support itself; its climbing powers are not required, and consequently are not developed.

So again, the leaves on the summit of a full-grown flowering plant, which would not require to climb any higher, were not sensitive and could not clasp a stick.We thus see how perfect is the economy of nature.

COMMELYNACEAE.--Flagellaria Indica.--From dried specimens it is manifest that this plant climbs exactly like the Gloriosa.A young plant 12 inches in height, and bearing fifteen leaves, had not a single leaf as yet produced into a hook or tendril-like filament; nor did the stem revolve.Hence this plant acquires its climbing powers later in life than does the Gloriosa lily.According to Mohl (p.

41), Uvularia (Melanthaceae) also climbs like Gloriosa.

These three last-named genera are Monocotyledons; but there is one Dicotyledon, namely Nepenthes, which is ranked by Mohl (p.41)amongst tendril-bearers; and I hear from Dr.Hooker that most of the species climb well at Kew.This is effected by the stalk or midrib between the leaf and the pitcher coiling round any support.The twisted part becomes thicker; but I observed in Mr.Veitch's hothouse that the stalk often takes a turn when not in contact with any object, and that this twisted part is likewise thickened.Two vigorous young plants of N.laevis and N.distillatoria, in my hothouse, whilst less than a foot in height, showed no sensitiveness in their leaves, and had no power of climbing.But when N.laevis had grown to a height of 16 inches, there were signs of these powers.