摘要:

SummaryAfter a brief historical introduction, attention is drawn to the importance of the correct sylviculturaj management of woodlands in the production of clear, disease‐free timber. Methods for the identification of the organisms which cause decay in timber are discussed, and the importance of pure culture methods is stressed. The principal contributions to the general descriptive work on timber decay in various countries are referred to, and it is pointed out how the usefulness of much of this work has been enhanced by the publication of books and monographs collating this scattered information. In a discussion of the physiology of wood‐rotting fungi the temperature relations and nutritional requirements, growth in relation topH, and moisture content of the substratum are considered. The unsatisfactory state of present‐day knowledge of the enzymes of wood‐decaying fungi is mentioned. Recent work on the metabolic products of wood‐rotting fungi is referred to briefly. The effects of decay on the physical properties of wood are described, i.e. on the density, strength and optical properties, etc. The mechanical strength of timber, especially the toughness, may be appreciably affected in a very early stage of decay before any measurable loss in weight has occurred. The chemical changes brought about during the decomposition of wood are discussed. Rot of timber can be classified chemically into two main types: brown rots, in which only hydrolysis of cellulose and other polysaccharides occurs, and white rots, in which all the constituents of the wood, including the lignin, are attacked in varying degrees and in which oxidizing as well as hydrolysing enzymes are present. The effect of fungal attack on the microscopic structure of wood is described, and the theories put forward to explain the penetration of the cell walls by hyphae are critically examined. Methods of determining the natural resistance to decay of timber in the laboratory are referred to, and the basis of natural durability in wood is shown to lie mainly in the nature of the chemical ‘extractives’ present in the wood. Recent advances are described in the use of water‐soluble wood preservatives which become fixed in the wood by the addition of a chromate to the treating solution, and the establishment of a British Standard method of test and the use of field tests are noted. Finally, attention is drawn to the great lag in the application to practice of recent advances in the study of timber decay and

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1943.tb00295.x

出版商:Blackwell Publishing Ltd    

年代:1943

数据来源: WILEY

摘要:

SummaryDuring recent years more work has been done on the practical value of minor elements than on their physiological function in the economy of the plant. There are still very few elements of which there is definite proof that traces are essential to growth.The importance ofboronin plant nutrition is firmly established. Large numbers of species are now known to suffer if the boron supply is insufficient and much commercial loss can be avoided by the judicious admixture of boron compounds with fertilizers. The value of Chilean nitrate is enhanced by its boron content. The part played by boron in the metabolism of the plant, however, still remains obscure.Withcopperattention has been focused on the value of copper salts in rendering fertile peat, or other highly organic soils, where they are brought under the plough. ‘Reclamation disease’ can be prevented by small dressings of copper sulphate before sowing. There is little direct evidence that copper is actually essential to the plant, though the assumption is generally made from the weight of indirect proof.Plants may play an important part as carriers of theiodinethat is so necessary for animals and human beings, but owing to technical difficulties it has not been possible to establish whether or not iodine is equally necessary to the plants themselves.The value ofmanganeseas an ameliorating agent for certain plant diseases has become increasingly evident, grey speck of cereals, marsh spot of peas and certain chlorotic disorders yielding to applications of manganese salts to the soil or as sprays on the foliage. The plant composition may be affected by treatment. Toxicity due to excess manganese may be controlled to some extent by variations in light intensity.Molybdenumin herbage is recognized as an important causal factor in some animal disorders as ‘teart’ disease. In plants excess produces characteristic morphological and cytological conditions, but there are indications that in some circumstances traces of the element may prove beneficial.Seleniumtoxicity in plants can be controlled by the judicious use of sulphur on seleniferous soils. As the absorption of selenium is reduced in this way, the risk of poisoning for animals feeding on the herbage is also minimized. Little definite evidence of benefit to growth due to traces of the element is yet available.Among other elements comparatively little is known as to whether any one or more may be essential for plant life.Isolated instances, particularly with fungi, indicate that such possibilities exist, but much work remains to be done before any assumption in this connexion can be made. The weight of evidence, indirect if not direct, with boron, copper, manganese, and zinc goes to indicate that each of these may play a vital role in plant development, though the exact conditions required and the extent to which each element is essential still remain subjects for invest

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1943.tb00296.x

出版商:Blackwell Publishing Ltd    

年代:1943

数据来源: WILEY

摘要:

SummaryIn order to colonize fresh waters, marine animals must maintain a total concentration of the body fluids above that of the external medium. This review is concerned with the probable course by which this independence was achieved during the evolutionary history of fresh‐water animals, so far as can be judged from ecological and physiological research. Many marine animals can endure a considerable dilution of the sea water without regulating their body fluids, the tissues being capable of normal function so long as the rate is not too great at which the body‐fluid concentration drops.Nereis diversicolorshows the beginnings of osmoregulation: a slight body fluid hypertonicity can be maintained when the external medium is dilute, and, when that medium fluctuates, the changes are sufficiently damped before they reach the tissues for the functioning of the latter to be unimpaired. By these means such animals have invaded brackish waters, and penetration of fresh waters may have been accomplished by some other groups through the immediate lowering of the blood concentration and subsequent development of a regulatory mechanism. On the other hand, some Crustacea (Erio‐cheir sinensisandTelphusa fluviatilis) have been able to invade brackish water thanks to a different regulating mechanism which prevents much diminution in body‐fluid concentration. Active absorption of salt from the medium through some part of the body surface (e.g. the gills) is an important component of this mechanism, though uptake from food via the gut must also contribute. The excretory organs, however, do not play a part, since the urine is isotonic with the blood. Most fresh‐water animals have a low blood concentration and the regulatory mechanism is assisted by renal reabsorption. Many are capable of actively absorbing ions from fresh water when the blood is artificially diluted, but it is not certain how far this is an important component of the mechanism in nature. On the other hand, several have remarkable powers of retaining salts, due no doubt to the high impermeability of most of the body surface. But it is possible that the organs concerned in absorption may also, by the same action, prevent loss of salt in very dilute media. Some animals however, are continuously dependent upon food for maintaining the salt level of the blood. The lower the blood concentration of a fresh‐water animal the lower the concentration of brackish water to which it can be adapted. This is no doubt partly due to irreversible adaptation of the tissues to a dilute body fluid, but the upper salinity tolerance limit may also be determined in part by the inability to obtain enough water for excretion from anything but a hypotonic medium. In the laboratory some marine animals have been adapted to fresh water and some fresh‐water species to sea water by altering the concentration of the medium extremely slowly. It seems that under such treatment the extension of the tolerance range is due to the development of a new mechanism not originally functional. Inland saline waters of low concentration have been invaded by some freshwater animals which cannot maintain their blood hypotonic to the medium, and, where the geographical situation permits, by some brackish water species. Adaptation to highly saline waters, however, requires a mechanism for hypotonic regulation, and all animals living in such waters are of fresh‐water origin. They can maintain their blood concentration at a level typical of fresh‐water animals. Some brackish water species are also capable of hypotonic regulation, but have not invaded waters of high salinity, probably because, unlike most fresh‐water animals, they do not produce stages resistant to the drought which is of common occurrence with i

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1943.tb00297.x

出版商:Blackwell Publishing Ltd    

年代:1943

数据来源: WILEY