Popular Science Monthly/Volume 46/January 1895/Correlation of Factors in Organic Growth
|CORRELATION OF FACTORS IN ORGANIC GROWTH.|
THE uniform co-operation of living cells in the vegetable organism appears less problematical to us when we know that these elements are connected by fine threads of living substance. These protoplasmic threads penetrate the cell walls; they immediately transmit the stimulus from cell to cell, and conduct it to a distance; and the continuity of the living substance in the whole organism is thus preserved. It formerly appeared otherwise, when the single living cell bodies were supposed to be completely separated by their walls, and these cell walls were thought to bring about the transmission. The physiological arrangements of plants have now become very much like those of animals, and nearly approach them in perfection. Very striking among the life expressions of organisms are certain processes which mutually influence and condition one another, and which we call manifestations of correlation. A particular condition in the organism invokes another, so brought about that a general balance in the functions is preserved, and is restored if disturbed. A red-beech tree growing in the open, where it is immediately exposed to the effects of the light, has small but relatively thick leaves. Red beeches, as undergrowth in the shadow of the woods, are distinguished by considerably larger but thinner leaves. The cause of this variation lies chiefly in the difference in the conditions of transpiration. The growing leaves in the isolated tree give out more vapor to the atmosphere than do those in the shade. The increased evaporation affects the structure of the leaf surfaces, and they are compressed, chiefly because less air space is found between their cells, and partly, also, because the cells turn perpendicularly to the surface instead of increasing in breadth. AU this increases the thickness of the leaf at the cost of its superficial diameter. This condition is immediately useful to the plant, because a thin and comparatively broad leaf surface would transpire too rapidly in an open situation, involving the tree in danger of drying out. In the shade, on the other hand, a large surface is necessary to give as much vapor out to the atmosphere as the life processes of the plant require; for evaporation promotes the accession of food-salts from the soil. These salts, dissolved in water, reach the plant, and are retained by it, while the water is evaporated. More rays of light fall upon a large leaf than upon a small one. In this view too, then, an enlargement of the leaf surface is more advantageous and useful in a feebly lighted situation. The intense light which falls upon a fully exposed beech may, in its greater intensity, perform as much. or even more work on the smaller leaf as the dimmer light of the shady situation on the larger one. An equivalent gain is realized by the lengthening of all the parts of the branch which takes place in the obscure situation. The leaves are thereby farther separated on the limbs, and do not cover one another. It is different in the isolated tree, where the leaves, even in a compressed situation, do not suffer for want of light. Hence, too, the great difference in the appearance of the conical crown of a solitary beech with its thickly compressed limbs and leaves, and that of the loosely spread out, umbrella-shaped undergrowth which the red beeches form in the shade of the wood.
A dry location promotes the same processes of growth as increased transpiration. The need of a plant in a dry soil is to reduce transpiration, and correlative processes are manifested through which that result is reached. The growing plant is so affected as to acquire a similar structure to that of a plant in a very sunny situation. The correlative operations in many plants take the form of giving a hairy covering to their leaves. A layer of air-retaining hairs diminishes evaporation. Hence the same plant may be hairless in moist ground, and in dry be covered with numerous hairs. The same is the case in plants from which the water supply is taken away. Superfluous growths are produced on the leaves and moderate transpiration. A slimy content is developed in the leaf cells of many plants, and serves to retain the moisture that is present within them. A dry location also generally promotes a greater thickening of the cells on the leaf surfaces, by means of which evaporation is made more difficult and prolonged. In the very cold, long-frozen soil of the arctic tundras plants have difficulty in obtaining the water they require; and in such situations, notwithstanding the real abundance of water, the same correlations in structure are found a*s in dry soil. The leaves are small and thick, and form slime within, and thicker cell walls on their surface. Similar rules obtain with plants in the saline soils of the steppes and the seashore Diversified conditions thus co-operate to produce the most favorable aggregate of life conditions for the plant.
The processes of transpiration in the plant appear adapted to introduce us into the difficult field of correlation, from the fact that it affords us an easier way than we can find in many other cases of looking into their mechanism. All the numerous instruments which perform the work in the organism interlock, mutually affecting and conditioning one another.
The researches of Julius Wiesner have shown that transpiration is the principal element in controlling the termination of the end bud. The vessels of our woody plants convey only a limited quantity of water to the unfolding leaves in spring. The growth of the leaf ceases at the moment when all the water brought up is claimed by these leaves. Correlative operations then come into play. The leaf parts in process of formation then assume the shape of bud scales, and all further advance in growth is interrupted. Very simple examination will show that this conclusion of the terminal bud is not caused by any deficiency of food. If the growing shoot is brought into a room saturated with moisture, the bud is not formed. Evaporation is arrested in such a room; there is no want of water; and therefore the mechanism is not unloosed which restricts the further development of the bud. Buds already closed may also be started in a new growth in a room saturated with moisture.
The closing of the buds in the axils of the leaves is conditioned on similar causes. These buds may be easily made to expand if the branch is cut off or stripped of leaves. The advantage to the plant of this property of restoring lost leaves is seen after every instance of devastation by caterpillars. If we cut off a young potato stem just above the ground, the parts growing beneath will at once come up out of the earth. They are the threadlike shoots which would otherwise swell at the ends and form potatoes. The want of incoming nourishment acts as a stimulant to the subterranean growth, and excites processes in them by which their nature is completely changed. Instead of, as before, growing straight down into the soil, the sprout directs its end upward, soon appears above the surface, produces green leaves instead of scales, and puts on the appearance of the ordinary leafy shoot. Corresponding in principle with the behavior of this potato plant is that of trees which give their side limbs an upward direction when they have lost their leader.
The internal and external changes which a fir limb has to undergo in order to become a leader are hardly less far-reaching than the transformation of an underground potato shoot into an aerial foliage stem. We perceive that the change of the course of the water and food stuff to the highest limb, which occurs after the removal of the leader, is the correlation force by which the metamorphosis is brought about; for these causes operate as stimuli to produce a change in the relations of light and gravity, by which a doubly differentiated organ is converted into one reacting on all sides alike—in a word, the whole living mechanism is profoundly transformed. When the metamorphosis of the lateral limb into a leader is completed, it can no longer be diverted from its upright position, but returns to it with the same persistency with which it formerly resumed the horizontal position.
Not less striking are other phenomena of correlation connected with the processes of transpiration, but which can be studied only under the microscope. The moisture which plants give out to the atmosphere escapes through, special openings in the epidermis, which are called stomata, or breathing pores. These openings are not visible to the naked eye. Fifty of these stomata may be counted on a square millimetre of leaf surface, and the number sometimes rises to five hundred in the same area. Each leaf is therefore provided with several million such openings. They expand or contract according to the necessity, and the correlation is so well adapted that the width of the opening is regulated so as to agree exactly with the existing conditions. They are usually closed at night, when a strong evaporation is not called for, because the salts which the water carries into the leaf can only be elaborated in the light. The transpiration being diminished at night, besides, by the lower temperature and the increased moisture in the air, the stomata can often remain open at that time without injury. This would be the case, for example, when the breathing process—which is likewise carried on through the stomata—is prolonged. The effect of the light is to open the breathing pores at dawn if they are closed, or to expand them. An increase in transpiration is now wanted, and it facilitates the exchange of gases by the assimilation of carbon. The opening fails to respond to the stimulus of the light if there is not enough water for the demands of the plant, when rapid evaporation would promote wilting. The active mechanisms of the plant react upon the external influences, and are in turn affected by them in the manner most advantageous to the plant at the given moment. The fact that the plant is not able to choose its reactions freely causes these reactions always to occur correctly according to the course of the external conditions.—Translated for The Popular Science Monthly from the Deutsche Rundschau.
"Buckland is persecuted," wrote Baron Bunsen of the eminent geologist to his wife in April, 1839, "by bigots for having asserted that among the fossils there may be a pre-Adamite species. 'How,' say they, 'is not that direct, open infidelity? Did not death come into the world by Adam's sin?' I suppose, then, that the lions known to Adam were originally destined to roar throughout eternity!"