Popular Science Monthly/Volume 83/September 1913/The Power of Growth in Plants
|THE POWER OF GROWTH IN PLANTS|
BOTANIST, MASSACHUSETTS AGRICULTURAL COLLEGE
IT has been a matter of more or less common observation from time immemorial that plants possess the power to overcome obstacles. Some species of trees are not particular where they grow if there is enough soil and moisture, their roots often seeking places where apparently insurmountable obstacles must be overcome. In spite of the doubt often expressed, there are on record many cases of trees lifting large weights; and in mountainous regions large boulders are often found displaced by roots growing among them. Some trees even lift themselves slightly from their original positions into the air, as is evident from the location of the root buttresses, which are often found exposed above the surface, sometimes for a considerable distance. An instance is known of a tree growing in the center of a millstone, which later completely filled the hole and actually raised the stone from the ground.
Brick and concrete sidewalks are often ruptured and curbings displaced by roots, due to their growth in diameter, and perhaps in some cases to the actual uplift of the tree trunk and roots. The writer has had under observation for many years a black birch (Betula lenta L.), one root of which has entered a fissure in a large boulder and is slowly but constantly lifting this enormous weight. The fissure is at an
angle of about 15 degrees. The vertical diameter of the root where it enters is only 4 or 5 inches, while its lateral diameter, owing to compression, is 18 or 20 inches, or more.
Careful measurements and specific gravity determinations would indicate that the weight of the boulder is about 18 tons, and this is 'annually being lifted higher and higher by the root growing in the crack.
The roots of trees often penetrate, and, as they grow, displace the foundation walls of buildings. We recall an old, heavy-timbered
colonial house which had one corner thrown considerably out of the vertical by the growth of tree roots under the foundation. In this case the roots must have lifted many tons in weight. In another instance a gentleman noticed that the stone in a walk leading to his residence had been displaced. He became alarmed and sent for the police, laboring under the impression that burglars were responsible for the displacement and were planning some deep plot against him. But on moving the stone, which weighed 80 pounds, three large mushrooms were discovered and the mischief was explained. Instances are known of mushrooms pushing up through hard tar walks two or three inches thick without the slightest difficulty or evidence of injury to their delicate tissues; and even seedlings often displace comparatively large masses of soil in pushing up through.
For several years we have been observing the rupturing of very hard concrete by ostrich ferns (Onoclea Struthiopteris L.). The concrete, Fig. 3. Showing epicotyl of seedling bursting through the soil. which is two and a half to three inches thick and composed of sand, tar and coarse gravel, acts as a watershed next a dwelling house. Along the edge ostrich ferns were some time ago planted in loam rich in organic matter, and have since been growing most luxuriantly, the stalks often reaching a height of six feet or more. Like most ferns, the underground stem or rhizome spreads out in all directions each year and thrusts up new fronds; and quite regardless of the apparently impenetrable covering, the rhizomes work their way under it and attempt to throw up new shoots. And not in vain, for the ferns appear to break through the concrete as easily as though it were so much putty. This rupturing occurs almost every spring when growth is active and the fronds unfolding. Sometimes the concrete is broken up where it joins the underpinning of the house and where it is more easily dislocated, and again the ferns come up through the middle.
The fronds which push themselves up through the concrete are necessarily more backward in unfolding than the unobstructed ones, although as a rule it requires only a week or ten days for them to break through. It required two years for one group of fronds to come through, though, as was evident from the constant upheaval of a part of the concrete one spring; but the next spring they succeeded in their attempt. The ease with which this breaking through is accomplished and the freedom of the ferns from scars and injuries are remarkable when the solidity of the concrete and the force needed to rupture it are taken into consideration.
Being interested in this phenomenon, we endeavored to learn approximately the power required by the ferns to rupture the concrete. In the experiment, some of the soil underneath was first excavated and a lever arranged in such a way that force could be applied in practically the same manner as was done by the ferns, i. e., a round piece of wood was placed on the end of the lever of the same dimensions as the undeveloped cluster of fern fronds. The fulcrum of the lever was one foot
from the point of contact with the concrete, and weights were placed on the other end of the lever at different distances, as the case required. Our object in this test was to ascertain how long it would take to rupture the concrete and to determine the amount of weight necessary to do it. It was not intended to apply force enough to cause an immediate rupturing of the concrete, or even in a few hours, but in perhaps ten or fifteen days—the same length of time usually required by the ferns. A number of tests were made, care being taken to have all the conditions as nearly like those under which the concrete was broken by the ferns as possible. A weight of 699 pounds broke the concrete in a few hours. Next a weight of 262 pounds was applied, which required ten days, while in still another test a weight of 189 pounds broke through in thirteen days. Other tests were made, but it is not necessary to give them here. A weight of 189 pounds, therefore, seemed to rupture the concrete in about the same time as was done by the ferns; and in our estimation this test represents a fairly good duplication of the fern phenomenon. If we consider the average cross section area of the six fern fronds and divide this by the total weight lifted, we find that the cells of the young fronds exerted about 35 atmospheres to overcome the resistance offered by the concrete. This we consider a very fair estimate, although from our other experiments we are led to believe that as high as 50 atmospheres are sometimes required to accomplish the work with the conditions under which the ferns were growing. The concrete was so hard that after it had been ruptured it was impossible to make any impression on the ragged edges except by the use of tools. The work was done by a slow and constantly increasing pressure on the under surface of the concrete, the principle being somewhat the same as in the straightening of teeth and bones, although in such cases the pressure is not increased.
At this point we might consider what growth is and how it is accomplished in a plant. Growth is defined as a stretching and fixation of the cell walls, accomplished by osmotic pressure characteristic of the solutions contained in the cell vacuole. In ordinary growth there is a pressure of 1 to 3 atmospheres on the cell walls—a fact which can be determined experimentally with some degree of accuracy. It is this pressure which gives plants their rigidity and freshness, and anything which destroys it, such as lack of water, causes the plant to wilt. Rapidly growing organisms—annuals and herbaceous plants, for instance—contain little mechanical or supportive tissue, and it is owing to the turgidity of the cells derived from osmotic pressure that they are able to hold their leaves and other organs in position. This could not be done without the exertion of considerable pressure, for their delicately constructed leaves and other organs often assume positions requiring a great deal of support. In trees and shrubs there is a large amount of mechanical tissue which supplies the necessary means for supporting the various members.
What is termed the "power of growth" can be determined by learning the amount of weight required to stretch a rapidly growing stem to its original length after the turgidity of the cells has been destroyed by placing them in plasmolyzing solutions, such as a 10 per cent, solution of potassium nitrate. The mean area of a cross section of a stem in millimeters, divided by the amount of weight obtained in grams, gives the number of grams per square millimeter of surface, and, as previously stated, there is usually obtained by this method a pressure of one to three atmospheres or more in the cell for ordinary growth.
While this osmotic pressure is common in ordinary growing organs, it does not necessarily follow that it constitutes the limit, since in the case of the ostrich fern previously referred to it was much higher. When growth is mechanically restricted or the organism has obstacles to overcome, the cell turgescence or osmotic pressure may be greatly increased owing to the resulting stimulus, and this is what occurred in the case of the ferns.
If a cross section of a stem is made and the bark split vertically, a noticeable shrinkage of the bark takes place, demonstrating a difference in tension between the outer and inner tissues. On the other hand, if longitudinal slices are taken from the outside of a common sunflower stem, they will shorten from 1 to 4 per cent, of their length, while the tissues from the center of the stem (pith) will lengthen from 1 to 6 per cent, when removed. It is clear from these observations that the various tissues of the plant are under tensions which may exhibit differences equal to 12 atmospheres or more. What is termed the "shearing stress" often becomes so great that the resistant cell walls are ruptured, a condition associated with great pressure in living cells. The injection of poisons into trees may likewise cause a rupturing of
the tissues owing to changes in the turgescence of the cells, and the splitting of melons in the field sometimes occurs from the absorption of an excessive amount of water into the inner cavity of the fruit. This increases the turgescence of the cells lining the cavity, and modifies the existing tissue tensions. The skin of the grape often cracks, possibly from the same cause.
It has been shown that the mechanical restriction of growth acts as a stimulus, inducing an increase in the osmotic pressure of the living cells, and in like manner, increased tensions may result in a much greater strength of the organism. It has been observed, for example, by Hegler, that a young sunflower seedling having an original breaking stress of 160 grams was able to maintain 250 grams after it had been stretched by a weight of 150 grams for two days, and later stretching of the stem by means of suspended weights over a pulley demonstrated that in a few days more its tensile strength was increased to 400 grams. This increase is correlated with thickness of the cell walls, a greater elasticity and the development of mechanical tissue.
The stimulation induced by the contact of tendrils and hook plants with objects is similar to that caused by stretching by weights. Experiments with the roots of various plants enclosed in plaster casts have shown large pressures. Pfeffer obtained osmotic pressures in the root cells of a common horse bean ranging from 5 to 19 atmospheres when the growth of the roots in length (longitudinal pressure) was mechanically restrained by the use of plaster-of-Paris casts, and from 2 to 6 atmospheres for the radial pressure of roots. The geotropically sensitive nodes of the wheat stem gave a pressure equal to 15 atmospheres when mechanically restricted. The maximum osmotic pressure m these cases would be obtained by a solution of potassium nitrate equal to about 5 per cent.
Colonel W. S. Clark's experiment with the lifting power of a squash, made in 1874 at Amherst, was one of the first attempts to learn the growing power of plants. This experiment attracted quite a little
attention at the time. One highly respected minister of the gospel had a drawing of the harnessed squash distributed among his congregation in tract form to illustrate the great moral principle that "If God in his providence has given such enormous power to growing vegetation to overcome difficulties, how much more will he give to you power to overcome the difficulties that may be in the way of your reaching the true end of all living."
This experiment was carried on in a greenhouse under the most favorable conditions, and by arranging an iron harness provided with a lever attachment the squash was found to raise 5,000 pounds. The squash was horticulturally known as the Mammoth Yellow Chile variety, and at the close of the experiment weighed 472 pounds. It is estimated that the squash developed over 80,000 feet, or about 15 miles of roots, an average of about 1,000 feet daily. From the data given in this experiment we have been able to estimate roughly the osmotic pressure of the cells, which might be supposed to be most active. but we have been unable to find that more than 22 atmospheres were involved. Professor Sachs, with the same data, estimated that the cell pressure developed was equivalent to a little more than one atmosphere.
Climbing and tendril-bearing plants, of which there are almost countless varieties, react to what is termed contact stimulation. Besides the many varieties which decorate our verandas and which are cultivated in our gardens for food, there are others with sensitive petioles Fig. 10. Showing growth of tissue over street sign placed on tree. The growth is restricted only at one point. The sign acts as a constant stimulus, including the callus to grow over it. (clematis and hook plant—Uncaria) which assist in anchoring the plant to supports. We have collected considerable data on the power displayed by tendrils and twining stems in clasping a support. Notwithstanding that the clasping results from the stimulation of the tendril, brought about by prolonged contact, the osmotic pressure does not ever appear to exceed the normal, only one to three atmospheres being found in these experiments. On the other hand, the effect of stimulation by contact in this case is to transmit the stimulus along the tendril, resulting in the formation of a spiral, and in most cases, if not all, the plant energy induced by the stimulus is directed towards the formation and modification of mechanical tissue, to render the union of the plant with the support more firm.
The formation of mechanical tissue in a tendril is well illustrated m the tendril of the common grapevine, and in various hook climbers. At first the tendrils of the grapevine are quite delicate and even edible, but later they become extremely hard and wiry. It would manifestly be a waste of energy from the economic point of view for tendrils to develop excessive clasping strength by means of an increased cell turgescence or osmotic pressure, since the clasping strength resulting from the normal turgidity or osmotic pressure of the cells is sufficient to answer all requirements. On the other hand, the increased production of mechanical tissue or a modification in the elasticity of the tendril is obviously of great advantage to it from the biological point of view. What is true for tendril plants appears to be true for climbing plants, such as the bean, as well as of plants with sensitive petioles, since there is no loss of energy displayed in the development of a superfluous osmoticin the cells for the mere purpose of increasing its clasping powers.