Exfoliation

“ It produces a superficial substance which plays the part of the epidermis and grows outwardly. For since the surface, subject to friction and exfoliation, has to be continually renewed, there must be a continual reproduction of an outermost layer from a layer beneath. That is to say, the contact of this deep-seated tissue with outer agencies, produces in it some approach towards that character which we find universally characterizes outer tissue. ”

“ This outer shell when heated is expanded, and so tends to peel off, or exfoliate, like the outer skin of an onion. The process is therefore described as exfoliation. In all rocks of homogeneous texture the continued action of this process results in convexly spherical surfaces, the material scaled off in the process remaining as a slope or talus which surrounds the projecting knob (Fig. 210) . Naked, these projecting domes rise above the rim of débris at their bases. Not a particle of dust adheres to the fresh rock surface—no dirt interferes with its glaring whiteness. ”

“ Towards the fifth day, however, after the sweating had entirely ceased, the complaint grew worse again. The spasms and paroxysms of suffocation returned, and they were succeeded by renewed eruptions of the exanthem; a decided improvement, however, shortly took place; the little pimples lost their redness, the miliary vesicles dried away, and at a period from the seventh to the tenth day recovery commenced under a general exfoliation of the cuticle. ”

“ In another type of the disease, formerly described as pityriasis rubra, the skin is pale red or violaceous-red, but is rarely thickened, continued exfoliation in the form of thin plates taking place. Its course is variable, lasting for years, with remissions.
An exfoliating generalized dermatitis is exceptionally observed in the first weeks of life (dermatitis exfoliativa neonatorum) , lasting some weeks, and in most cases followed by recovery. There are no special constitutional symptoms, the fatal cases usually dying of marasmus. ”

“ The recesses were formed by a process called differential weathering or exfoliation. Moisture seeps into the cracks behind the surface of the softer layers of limestone. When the water freezes it expands, cracking off thin layers of rock. ”

“ The term _exfoliation_ is sometimes employed to indicate the separation or throwing off of a superficial sequestrum. The edges and deep surface of the sequestrum present a serrated or worm-eaten appearance due to the process of erosion by which the dead bone has been separated from the living. ”

“ The Flesh must be open'd to give Passage to the Sanies that runs out of the ulcerated Bone, to the end that Exfoliation may be procur'd; and if the Ulcer hath not as yet laid open the Bone on the outside, the Trepan ought to be apply'd; but the Ulcer or Caries must be afterward handled, as we have even now declar'd. ”

“ It is true that often very distinct layers are separated from the living bone, but these layers are only the product of exfoliation itself. Then in fact the bone dies on the surface; the superficial vessels receive no more blood; this fluid is stopped under the portion deprived of life, the exhalation of the phosphate of lime ceases there, every kind of sanguineous, exhalant and absorbent vessel is destroyed; a slow inflammation, with suppuration, comes on, and fixes the line of demarcation ”

“ These plants belong mostly to the same group as the moulds, and they are especially apt to attack the skin. They grow in the skin, particularly under the hair, and may send their threadlike branches into some of the subdermal tissues. This produces irritation and inflammation of the skin, resulting in trouble, and making sores difficult to heal. So long as the plant continues to grow, the sores, of course, can not be healed, and when the organisms get into the skin under the hair it is frequently difficult to destroy them. ”

“ Bark-boring—Wood-boring—Wood fungi—Leaf-miners—Pith flecks—Erosions. Skeleton leaves—Irregular erosions—Shot holes. Frost cracks—Strangulations—Spiral grooving.
Natural wounds are produced in a variety of ways during the life of the plant, and, generally speaking, are easily healed over by the normal process if the area destroyed is not too large, and the parts remaining uninjured are sufficiently provided with foliage, or with supplies of food-materials stored up in the roots, rhizomes, medullary rays, etc., to feed a vigorous callus. ”

“ The Signor Nardo recounts the results of experiments made on his own person in the transplantation of hair, and relates, that by transplanting quickly a hair, with its root, from a pore of his head, into a pore of his chest, easily to be accomplished by widening the pore somewhat with the point of a needle, introducing the root with nicety, and exciting within the pore itself, by friction, a slight degree of inflammation, the hair takes root, continues to vegetate, and grows; in due season changes colour, becomes white, and falls. ”

“ As the root continues to extend, parts behind the tip cease to be effective because, as soil particles in direct contact with these tips and hairs dry out, the older roots thicken and develop a bark, while most of the absorbent hairs slough off. This rotation from being actively foraging tissue to becoming more passive conductive and supportive tissue is probably a survival adaptation, because the slow capillary movement of soil moisture fails to replace what the plant used as fast as the plant might like. ”

“ It is often said that the cause of defoliation in trees arises from a deficiency in the leaves of the power of absorbing moisture. The delicate pores by which the life of the leaf, so to speak, is sustained, and which ought to be vigorously inhaling carbonic acid and giving forth oxygen, alternately receiving and yielding moisture, become clogged, and the leaf fades and falls, just as a human being dies when his respiratory organs cease to act healthily. ”

“ A culture may be prepared from the sweating bacteria by taking the hair from the skin when it “slips”—that is, when it can be removed by simply pulling it. The root portion of the hairs are then cut off, and about 10 grm. carefully digested in 100 c.c. of water at a temperature of 32° to 35° centigrade for several hours. The liquid portion is then poured off, and used for inoculating 10,000 c.c. of a nutrient medium of the character hereinafter described. ”

“ The cooling properties of leaves would be recognised by the most primitive peoples; and as a cut or other wound, by the process of inflammation, at once begins to burn and throb, a cooling leaf would be the most natural thing to apply. Some leaves which possess styptic and resinous properties would staunch bleeding very effectually, and the mere act of binding round the cut an application like a leaf would serve to draw together the edges of the wound, and afford an antiseptic plaster of the most scientific nature. ”

“ By this method the entire body is hermetically sealed (just as our New England forebears used to seal their preserves and jams with paraffin) and the skin is kept immaculately clean.
As soon, however, as the Spring sun has ameliorated the low temperature the native feels that it is time to slough his oily protection. Nature demands that his pores come up for air. This is accomplished by exposure to the sun's rays. The wax and substrata rapidly liquefy and are easily scraped-off with curved bone knives admirably adapted to the work in hand. ”

“ Modifications of the form of leaf and stem are alike in that they lessen transpiration by a reduction of the amount of surface exposed to the air. Structural adaptations, on the other hand, bring about the protection of epidermal cells and stomata, and often internal cells also, from the factors which cause transpiration, or they anticipate periods of excessive transpiration by the storage of water in specialized cells or tissues. In certain extreme types the epidermis is itself modified for the absorption of water vapor from the air. ”

“ Although an incision is always the most certain, and it is uniformly practiced, roots will in many varieties strike easily from the buds; and a common operation in France is simply to peg down the branches in the soil, without any incision; in some cases, they give the branch a sudden twist, which will break or bruise the bark, and facilitate the formation of roots. ”

“ Experienced method of healing Wounds in Trees.—This method consists in making a varnish of common linseed oil, rendered very drying, by boiling it, for the space of an hour, with an ounce of litharge to each pound of oil, mixed with calcined bones, pulverized and sifted, to the consistence of an almost liquid paste. With this paste the wounds of trees are to be covered, by means of a brush, after the bark and other substance have been pared, so as to render the whole as smooth and even as possible. The varnish must be applied in dry weather, in order that it may attach itself properly. ”

“ The protoplast lines the cell, and the central part of the root hair consists of a large vacuole filled with cell sap. The wall of the root hair is composed of cellulose, and the outermost part is frequently mucilaginous. As the root hairs develop, they become bent, twisted, and of unequal diameter, as a result of growing through narrow, winding soil passages. During their growth, the root hairs become firmly attached to the soil particles. The walls of root hairs give an acid reaction caused by the solution of the carbon dioxide excreted by the root hair. ”

“ I will next instance the "perfoliates," plants not confined to any one order, but alike in having a stem which passes midway through the leaf or pair [Pg 87] of leaves, a most engaging curiosity of structure. It is by this peculiarity that the yellow-wort, a gentian with glaucous foliage and blossoms like "patines of bright gold," mainly wins its popularity. But the quaintest of perfoliates is the hare's-ear, or "thorow-wax," as it used to be called, of which, as Gerarde wrote, "every branch grows thorow every leaf, making them like hollow cups or saucers." ”

“ They have skin cells, but no pores. Out of their skin cells grow their most interesting and important parts. These are called root hairs. They are made of cells lying next each other, like other hairs, but they do all the sucking up of food materials for the whole root. These root hairs draw the water and other food out of the soil for the use of the plant, and the rest of the root only stores it up and conducts it to the stem and leaves above and anchors the plant to the ground. ”

“ Surely, nothing with which plants are provided is so important to them as this delicate membrane of the root hairs which, while allowing osmosis to act in a one-sided fashion, preserves to the plant the sugary liquid that alone makes the absorption of soil water possible.
As root hairs are very much alive and work constantly, they must be provided with air, without which no living thing can exist. And here, again, it seems as though nature, with almost uncanny foresight, had deliberately planned for this requirement of roots. ”

“ The first operation to which they are subjected is depilation, which removes not only the hair but also the scarf skin or epidermis. When the goods are sent to the limes for depilation they are, first of all, placed in an old lime, highly charged with organic matter and bacteria. It is the common belief that the lime causes the hair to loosen and fall out, but this is not so; in fact, pure lime has the opposite effect of tightening the hair. ”

“ The Epidermis or skin of leaves and all young shoots is best seen in the foliage. It may readily be stripped off from the surface of a Lily-leaf, and still more so from more fleshy and soft leaves, such as those [Pg 144] of Houseleek. The epidermis is usually composed of a single layer, occasionally of two or three layers, of empty cells, mostly of irregular outline. The sinuous lines which traverse it, and may be discerned under low powers of the microscope (Fig. 487) , are the boundaries of the epidermal cells. ”

“ Although, as shown in the experiments with wheat, the short roots in dry soil were more densely covered with root-hairs than the much longer roots in moderately moist soil, subsequent closer investigation shows that the total quantity and area of root-hairs is less in the former case than in the latter.
The greatest number of root-hairs are developed on roots which are growing at their best: too much moisture may prevent the formation of root-hairs: too little may induce dense growths of root-hairs locally, but the total number is reduced. ”

“ Epidermis.—The outer or epidermal cells of leaves do not bear chlorophyll, but are usually so transparent that the green mesophyll can be seen through them. They often become very thick-walled, and are in most plants devoid of all protoplasm except a thin layer lining the walls, the cavities being filled with cell sap. This sap is sometimes coloured, as in the under epidermis of begonia leaves. It is not common to find more than one layer of epidermal cells forming each surface of a leaf. The epidermis serves to retain moisture in the leaf and as a general protective covering. ”

“ The mechanical tissue of the stem is arranged in the form of solid or hollow columns in order to sustain the enormous weight of the branches. In roots the mechanical tissue is combined in ropelike strands, thereby effectively resisting pulling stresses. The epidermis of leaves subjected to the tearing force of the wind has epidermal cells with greatly thickened walls, particularly at the margin of the leaf. The epidermal cells of most seeds have very thick and lignified cell walls, which effectively resist crushing forces. ”

“ A microscopical examination of the tissues of the plant shows them to be comparatively simple, this being especially the case with the fibro-vascular system.
The epidermis of the leaf is readily removed, and examination shows it to be made up of oblong cells with large breathing pores in rows. The breathing pores are much larger than any we have yet seen, and are of the type common to most angiosperms. ”