Part 25

No structure in plants seems more wonderful, as far as its functions are concerned, than the tip of the radicle. Lightly pressed or burnt or cut, it transmits an influence to the upper adjoining part, causing it to bend away from the affected side. But more surprising, however, is the fact that the tip can distinguish between a slightly harder and softer object, by which it is simultaneously pressed on opposite sides. Let the radicle be pressed by a similar object a little above the tip, and it will be noticed that the pressed part does not transmit any influence to the more distant parts, but bends abruptly towards the object. Perceiving the air to be moister on one side than the other, it likewise sends out an influence to the upper adjoining part, which deflects towards the source of the moisture. When excited by light, the neighboring part bends from the light; but when excited by gravitation, the same part bends towards the centre of gravity. In almost every instance the ultimate purpose or advantage of the several movements can be clearly perceived. Two, or perhaps more, of the exciting causes often act simultaneously on the tip, and one conquers the other, doubtless in accordance with its importance for the life of the plant. The course pursued by the radicle in penetrating the ground being determined by the tip, has acquired for it the diverse kinds of sensitiveness which it possesses; and it is hardly an exaggeration to assert that the tip of the radicle thus endowed, and having the power to direct the movements of the adjoining parts, acts like the brain of one of the lower animals, which organ, seated within the anterior end of the body, receives impressions from the sense-organs, and directs their several movements.

In animals possessed of a nervous system, contractions only follow stimuli, which are carried to the contractile elements by nervous threads, the internal energy representing the external stimulus being called nervous energy or neurism. But where a nervous system does not exist, as is the case in some low animals and in all plants, external stimuli must be justly supposed to be converted into the same form of energy, which in such organisms has a general circulation throughout the contractile protoplasm. The attainment of some position, favorable for the procurement of relief from some unpleasant sensation, or the acquisition of some agreeable one, or for both, is the important thing directly subserved by such movements in the generality of animals. While we have the best of reasons for believing this to be true in the vast majority of animals, because fundamentally their structure is similar to our own, yet the inference that the same is true of the lowest forms of life is justifiable until it is proved to be mistaken.

Whatever be the nature of any movement, whether the projecting of portions of its own body-substance as pseudopodia in the primitive animal, the movement of flagella or cilia in more specialized forms, or the turning of the radicle of a plant-seedling in overcoming some obstacle, there is no resisting the conclusion that the functions of these organs, when once called into existence, are due to stimuli not unlike those which affect the motions of the limbs of the higher animals, and that the preliminary to all such movements, which are not automatic, is an effort. And as no adaptive movement is automatic the first time it is performed, effort, therefore, may be regarded as the immediate source of all movement. Now, effort is a conscious state, and implies a sense of resistance to be overcome. But when an act is performed without effort, resistance has been overcome, and the mechanism requisite for its performance has been completed. Automatism has now been reached. New movements, in their incipiency, necessarily meet with resistance. How this resistance is overcome, there seems to be some diversity of opinion among physiologists and metaphysicians, but it is generally believed that some such mental state as a sensation or a desire, which may or may not stimulate a natural process as an intervening element in the circuit, is concerned in its subduement. That sense-perceptions are stimuli to the immediate appearance of structural changes or movements is shown by the production of color-changes in animals through changes in the condition of the organs of sight and in the bending of the radicle of a seedling-plant a short distance above its tip in obedience to a communication from the tip of a sensation of hardness, caused by contact with a stone experienced in its downward progress in the ground.

New conditions bring forth new acts in animals. No one can deny this statement, as instances of its truth are too frequent to believe otherwise. That such may be predicated of plants, which have not the ability, as a rule, to meet with new conditions by reason of their being affixed to the soil, very few persons are willing to admit; but there is no getting away from the fact. The tip of the radicle of a plant not only has the power, acting as a brain, as it would seem, of guiding the root out of the reach of an obstacle that would be injurious, or in the direction of water when it would be an advantage, but a tendril has also the ability, in obedience to some inherent force, of making its way to a support that has been purposely placed in the near distance for its especial benefit. No external agencies, which the materialistic naturalist has devised for accounting for the movements of plants and low types of animal existences that are devoid of a visible nervous system, can possibly explain these movements, which are only explicable on the theory that nervous energy may be elaborated and be distributed without such a system by and through the general mass of the plant or animal, or by and through such parts as may be necessary to its good.

No one who has experimented with the Droseras or Sundews, can have failed to observe the extreme sensitiveness which resides in their leaves. That these plants manifest a comparatively high order of consciousness, there can be no question. Try them with insects, or rare bits of meat, as articles of diet, and in a few hours, if vigorous leaves have been experimented with, the leaves will have folded around the food and commenced their curious process of assimilation. Mineral substances, such as bits of chalk, magnesia and small pebbles, have no such effect. They seem to ignore these things, just as an intelligent animal would if they were placed by its side. Some experiments made by Mrs. Treat, several summers ago, go far to confirm the statement that plants are endowed with some sort of consciousness. _Drosera filiformis_ was the species used in her experiments. Some living flies were pinned one-half an inch from the leaves, but near their apical extremities. In forty minutes the leaves had perceptibly bent toward the flies, and in little more than an hour had reached the prey, the legs of the latter being entangled and held fast by the tentacles of the leaves. Next, the flies were removed three-quarters of an inch further from the leaves, but the latter, even though bent away from the direction of the light, failed to reach them at this distance. What was it that induced the leaves to stretch in the direction of the flies? Had the sun been shining from that side, it might be said that the movement of the leaves was influenced by its light and heat, for plants as a general rule turn toward that part of the heavens where these energies are the most effective. It cannot be that they were produced by some emanation of moisture from the bodies of the flies, or by any influence that might be exercised by the vibratory movements of their wings. No vain imaginings of such character will suffice for their explanation. The energy necessary to explain this phenomenon must come from within the leaves themselves. There was felt within them a desire for food, and it was this desire that led the leaves to bend away from the light and in the direction of the objects whose presence created in them that sensation. But how they were able, in the absence of any visible sense-organs, to determine the presence of these objects, is difficult to surmise. That they are sensitive to contact is generally conceded. And in them, no doubt, the sense of touch is keenly developed. Granting this to be the truth, then they see, as a blind man sees, by the sense of feeling. Currents of air, established by the vibration of the insect’s wings, impinging upon the epidermis of the leaves, affect the cells beneath, and a nervous influence is started, guided by some central agency, of which we know nothing, causing the leaves to bend in the proper direction. But why the leaves do not thus bend when impinged upon by currents other than those produced by insects, I am unable to say. Even as a blind man, though deaf, is able through the sense of touch to discriminate moving objects by the currents of air they excite, so it may be presumed that the leaves of Drosera are endowed with the same wonderful and intelligent capacity. Such a feeling once experienced would be apt to be known again, for it would become fixed in consciousness by a process of memory. That Drosera, whose habits are more animal-like than plant-like, must occupy a high position in the scale of vegetable life, there can be no reason to doubt from what has been said, and this assumption receives a most remarkable confirmation from the fact that there are evidences, not apparent however, of a sort of nervous system in its make-up, as shown by the discovery of Darwin that by pricking a certain point in a leaf one-half of its substance becomes paralyzed.

Wonderful as these facts are, yet they are not more so than some recent discoveries made by Stahl while studying the simple movements and physical conditions of certain low plants called Myxomycetes. In their young stages these plants wander from the parts of the deposit on which they are creeping, and which are gradually drying up, toward those which are more moist. It is possible, by bringing moist bodies in proximity to any ramifications, to produce pseudopodia, which lift themselves from the deposit, and soon come into contact with the moist object, so as to enable the whole mass of the plasmodium, that is, the large, motile, membranous protoplasmic body formed by the coalescence of the swarm-spores of the Myxomycetes, to migrate thereon. But on the entrance of the plasmodia into the fructifying condition, the Myxomycete quits the moist deposit, technically called the substratum, and creeps upwards on to the surface of dry objects. Unequal distribution of warmth in the substratum and unequal supplies of oxygen and chemical substances soluble in water also cause locomotion in these strange organisms. Let the plasmodia come into contact on one side with solutions of saltpetre, carbonate of potash or common salt, and they at once withdraw from the dangerous spot; but an infusion of tan, or a dilute solution of sugar, causes a flow of the protoplasm and an ultimate translocation of the entire plasmodial mass towards the source of nourishment. Some solutions have an attractive or repulsive effect, but this is in accordance with the degree of their concentration. Unlike what is so natural to plants in general, the Myxomycetes seem to have an aversion to light, as shown by their disposition to withdraw from its presence.

How such tender structures as the Myxomycetes, which are destitute of every kind of external protection, are enabled to carry on their existence, the knowledge of the remarkably delicate reaction of their plasmodia under external influences prepares us to understand. Plasmodia, which are not yet ripe for reproduction, are kept in the moist substratum by their peculiar affection for moisture and utter dislike of the light. But within the darkness and moisture of the substratum the plasmodia do not necessarily remain in one place, for the differences in the chemical composition of the substratum cause continual migrations. Nothing more remarkable can be said of the plasmodia than that they have a wonderful faculty of avoiding harmful substances, and, traversing the substratum in all directions, of taking up the materials they require for food and growth. When, however, their internal changes have advanced so far that the plasmodia are approaching the fructifying condition, they are brought by their dislike for moisture, which now sets in, from the moist ground of forest or wood which they affect to the surface, where they creep up various upright objects, frequently not doing more than forming rigid reproductive capsules at some height from the ground. If, however, the substratum becomes gradually colder, as is the case in autumn, a change which sets in at the surface moving downwards, then the plasmodia migrate into deeper regions still having a higher temperature; but when the cooling proceeds very gradually, which especially happens in large tan-heaps, the plasmodia may in their migration attain considerable depths, where they then change into sclerotia, which are hard tuberous substances, resembling the tubers and bulbs of flowering plants. If, however, the temperature begins to ascend, the sclerotia again germinate, and movement takes place from the deeper and cooler parts to the upper already named.

Thus we see, in the locomotion of the Myxomycetes, extremely interesting cases of movements due to stimulation. Light, heat, moisture and gravitation are, in general, stimulus-movements, and ultimately all growth depends on stimulus-movement, the most primitive kind of protoplasmic movement. No causes other than those which actuate higher organisms can be discerned to account for this lowest type of organic movement. What form of inorganic energy can be cited of sufficient potency to cause the organism to change, and without regard to gravitation or any known form of attraction or repulsion, its position in obedience to stimuli acting for its self-preservation? There is none. In the Fuligo, or Tan Flower, a most remarkable example of designed movement has been observed. This form will, according to H. J. Carter, in its early amœbula stage, when isolated from the sawdust and chips of wood among which it has been living, adapt itself to the water of a watch-glass, or any other shallow vessel, in which it may happen to be placed. But, if the watch-glass be placed upon the sawdust, then it will make its way over the side of the glass to get to the sawdust. Here is probably shown a sense-perception of the presence and position of the tan-bark, as well as a feeling of desire to go to it. May not this desire have been due to a sense of discomfort induced by the surrounding water, or to the calling up in memory of some superior comfort associated with the tan-bark?

Man in his self-complacency thinks that he knows the plants about him. It is true that he has noted their form, their anatomy, their color and their resemblances and differences, but how few have studied them in meadow and woods by the light of a lantern at night or by the silver rays of the moon. One feels on such an occasion as though he had stepped from his threshold upon a foreign soil. Folded leaves and strange sleeping forms will be found to confront you in every direction. Of the nature of the nocturnal movements of plants, as well as their varied and curious attitudes, both in leaves and flowers, much speculation has been rife among botanists. In many flowers the night attitudes have been conclusively shown to have relation solely to their fertilization by insects; but the drooping night attitudes of the leaves were supposed to indicate an aversion to moisture, many plants seemingly verifying the conjecture by the assumption of the same position during rain as in the dew. But when the same pranks were played on a cloudy day or a dewless night, the explanation had to be abandoned. With the clovers, the nocturnal positions of the heads seem to be assumed only in the darkness, and this invariably, dew or no dew, while the leaves appear to revel in the rain, remaining freely open, their chief concern being the protection of the young blossom-clusters.

Were our eyes sharp enough we might discern a certain strangeness in the nocturnal expression of every plant and tree. But in no tree is this expression so remarkably emphasized as in the locust, a member of the same leguminous order of plants with the clover. These trees are especially noted for the pronounced irritability of their leaves, and odd nocturnal capers, whose seeming vital consciousness has induced some authorities to place them at the extremity of their system, in contact with the limits of the animal kingdom. How strange the pigweeds look at night! Their upper leaves, which during the day had extended wide on their long stems, now incline upward against the stalk, enclosing the tops of the younger branches, but still older plants are seen with leaves extended much as at mid-day, but nearly all turned edgewise by a twist in the stem. Circling in a close curve, the creeping-mallow blossom now ignores her proud array of cheeses, and the oxalis flower has forgotten her shooting pods to keep the vigil, closed and nodding upon her stem, while her leaves masquerade in one of the oddest disguises, their three heart-shaped leaflets being seen reflexed and adjusting themselves back to back around the stem with many contortions. Whatever the function of this strange nocturnal movement may be, and it is still a matter of dispute with botanists, one thing we are certain about, that is, its essential condition to the life of the plant, careful experiment having demonstrated, according to one authority, that “if the leaves are prevented from so regulating their surface, they lose their color and die in a few days”--a fact which Darwin has just as conclusively shown to be the case with other plants.

Flowers that bloom by night could hardly be suspected of that vanity which Rhodora has been made to confess by Emerson in his beautiful lines to this flower. Our evening primrose does not bloom in the dark hours for mere sentiment or moonshine, but from a nature which lies, figuratively speaking, much nearer her heart. “Often when the nights are very dark,” says an old writer, “her petals emit a mild phosphorescent light, and look as if illuminated for a holiday. And he who does not fear to be out in her mild and lovely haunt may see a variety of nocturnal ephemeræ hovering around the lighted petals, or sipping at the flowery fountains, while others rest among the branches or hurry up the stems as if fearing to be too late.” From the first moment of her wooing welcome it would seem that our evening primrose listens for murmuring wings, and awaits that supreme fulfilment with joyous expectancy, for it will invariably be found that these blossoms, which open in the twilight, have adapted themselves to crepuscular moths and other nocturnal insects, a fact which finds a striking illustration in the instances of very long tubular-shaped night-blooming flowers, like the honeysuckle and divers orchids, whose nectar is beyond the ability of any insect but a night-flying hawk-moth to attain. True, it is, that in other less deep nocturnal flowers the sweets could be reached by butterflies or bees if the blossoms were left open. But the night-murmurers receive the first invitation, which, if accepted, leaves but a wilted, half-hearted blossom to welcome the sipper of the sunshine. This beautiful expectancy, somehow or other, determines the limit of its bloom. However, in the event of rain or other causes preventive of insect visits, the evening primrose will remain open for the attention of the butterflies during the ensuing day, when otherwise it would have perceptibly drooped, and extended to them but a listless welcome. Most strikingly may this fact be seen illustrated in a spray of mountain-laurel. For nearly a week have I observed in my house these blossoms lingering in patient expectancy, when the flowers on the parent shrub in the woods had fallen several days before, their mission in life having been fulfilled. In the house specimens the radiating stamens, which are naturally dependent upon insects for their release, and the consequent discharge of the pollen, remained in their pockets on the side of the blossom-cup, a support, as it seemed, for the bracing up of the corolla upon its receptacle. But when the operation of releasing the stamens was artificially consummated, the flower-cup soon dropped off or withered upon the peduncle.

Not mainly has the writer, in attributing a phosphorescent quality to the evening primrose, followed the license of fancy, for, if scientists are to be believed, the regular luminous glow of this and other nocturnal flowers has long attracted the attention of the curious, and positive qualities of inherent light have been accorded in many instances. It is true, as one authority asserts, that “the evening primrose is perfectly visible in the darkest night,” from which fact phosphorescent properties have been ascribed to it. Many well-authenticated cases are on record of luminous, electrical, lightning-like phosphorescence playing about flowers, the daughter of Linnæus having been the first one to note such an interesting phenomenon. Similar flashes or corona have been observed in nasturtiums, double marigold, geraniums, red poppy, tuberose, sunflower and evening primrose. According to various authorities, and it would be a rash and presumptuous commentator who would dare to challenge such an array of competence, many beautiful surprises await the traveller among the dewy shadows. Whoever has made such a journey will not only return with the consciousness that he has doubled his possessions, but that he has also explored a new world--a realm which he can look in the face on the morrow with an exchange of recognition that was truly impossible yesterday.