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Thorns and Prickles


In the midst of an old pasture stands a stunted apple tree. By all the signs, it is a close relative of the thrifty trees that grow in the neighboring orchard. They are large because they grow in fertile soil and are given careful tillage. But this ugly dwarf sends its roots into a hard crust whose nourishment is largely stolen by the mat of grass roots. The twigs have had little encouragement to grow. Every ambitious shoot has paid dearly for its temerity. It has become a sweet morsel under the tongue of some hungry cow. Starved and browsed to the point of utter discouragement the tree stands - a most unlovely shape, with stubby twigs hardened and sharpened into ugly spurs that look like thorns.

Over in the orchard the twigs are long and lusty, with ample leaves and plump buds; and there are no thorns at all. The fat and lean kine of Pharaoh's dream were not more like and more unlike than the fat and lean apple trees we are considering. Nevertheless the pasture tree does grow, if very slowly, and there comes a turning-point in its life. Its roots find better and deeper feeding ground. Among the young shoots there is one that is out of harm's way. It is in the very middle of the top. The cows come by and are cheered by Wicked-looking the sight of it. They lean toward it, but the longest neck is thorns of not quite long enough. The thorny twigs make a stubborn defense. One last inch of distance cannot be compassed by any patient, yearning tongue. The shoot mounts up and out of danger. Its twigs grow soft and succulent. One can almost fancy that the parent tree is neglecting the lower branches in order to give this youngest one the best of everything.

Plainly, apple twigs grow soft and leafy when well fed. Poverty and abuse make them crabbed and thorny. Stunted twigs are the products of "hard times." The carrying of weapons is a habit to which the trees are driven by adversity, and which they abandon as soon as "good times" return. On the other hand, there are certain trees which habitually bear thorns. One of them is the honey locust. Above the opening leaf a sharpened point comes out of the twig. At the end of the season the leaf falls, but above it is left standing the wicked-looking three-pronged thorn, sharp as a needle and hard and smooth as if enameled. It is not uncommon to see a honey locust tree with trunk and limbs fairly bristling with these thorns, the largest approaching a foot in length.

What are these thorns? Are they branches, assigned to special duty, and properly uniformed for their work? It is useful and interesting at this point to bring in for comparison a branch of one of the hawthorns. Near the end of the branch is a half-grown leaf in whose axil arises a slender thorn. It is green and soft, and set with a half-dozen tiny leaves. Farther down are bigger thorns, set in the axils of full-grown leaves. These thorns have hardened. Below, on older wood, are thorns of larger size. Some of them have leafy shoots on their sides. Here we have a series of thorns, the youngest of which show the leaf-bearing habit of the twig, the oldest ones, the twig-bearing habit of the branch. The honey locust thorn shows the branching habit. Tear off any of these thorns, and you find them attached to the stem just as the twigs are.

On evidences like these the botanist bases his belief that thorns are branches, hardened, pointed and destitute of leaves, gradually modified by the plant to serve its special needs. The beginning of such modification has been seen in the pasture apple tree. The process has progressed much farther in hawthorn and honey locust, where the branch has assumed a kind of disguise - the livery of its special office.

The common locust bears at the base of its leaf two sharp points, and each leaflet repeats this peculiarity by having two tiny guardsmen of the same kind at its base. But these points, though they persist and some of them grow large and strong, do not rise from the wood of the branch as do thorns. They come off with the bark. Hence they are prickles such as grow on bushes and raspberry canes. They are mere outgrowths of the bark. Certain spines are evidently modified leaf ribs. The holly that we see at Christmas has the edge of its leaf contracted between spiny points. The barberry shows all the gradations between leaf and spine on the same twig.

In arid countries the vegetation tends to be leafless and spiny. Not cacti alone but other plants have their surfaces reduced and hardened or otherwise protected against loss of moisture in the hot, dry climate. In more humid regions the same species of plants have their spines dilated into leaves.

Each modification of bark or leaf or branch into prickle or spine or thorn is an expression of the varying needs of plants, and is the final result of Nature's attempt to adapt plants to their surroundings. We cannot say that thorns exist for defense against injury by animals, for we have no absolute proof of it. Yet it seems to us an obvious inference. We must avoid jumping at conclusions. Nothing is easier than for us to deceive ourselves by unconsciously projecting our experience into nature. Our point of view is not the same as the plant's. It is easy to say that cacti have thorns to protect them from being browsed by cattle. Scientific research, however, shows that the thorns Of cacti are probably mere incidents to the contraction of the whole plant body, the main enemy being drought rather than browsing animals.

The truth in these matters of adaptation and specialization of parts can be gotten at only by a thorough study of each plant's actions under varying circumstances. The student must come to his work with his mind free from preconceived notions and theories on the subject. If, in many cases, he finds his researches unfruitful, there is always this question to ponder upon: "Is it reasonable to expect Nature to reveal to me in a few months or years the stages by which, through centuries, perhaps, she has been making perfect the adaptation of this plant to its present environment?"