摘要:

Summary1. ‘Light’ covers a variety of related measures of radiant energy: the particular usage must be defined for each investigation.2. Light in the open varies in intensity and spectral composition, both with time and over space. These variations can usually be formulated mathematically and used to predict conditions in plant communities.3. Light may be measured directly with radiometric instruments, responding equally to all wavelengths within a certain waveband, or with photometric instruments, whose response approximates to that of the human eye. The use of the former is desirable whenever possible, as it simplifies subsequent interpretation of the results.4. Photographs covering an entire hemisphere can be used to estimate the contribution of diffuse and direct light separately under many circumstances. They are a great help with the interpretation of many forms of measurement.5. If the percentage of light transmitted by a plant community is to be treated as constant, diffuse and direct light must be treated separately. Even so, it is only the average transmission over considerable periods that is reasonably constant. The pattern of diffuse light distribution in the community is very different on clear and on cloudy days.6. In general, most diffuse light penetrates the plant community near the zenith. However, on clear days the aureole of light near the sun shining through a low‐altitude canopy gap may entirely alter this relationship.7. The distribution of direct light can be analysed from tracing the path of the sun across the canopy. The sun shines unobstructed through gaps of an angular width greater than 1/2.8. In some herbaceous communities the logarithm of relative intensity of diffuse light inside and outside the community is approximately proportional to the cumulative leaf area per unit area of ground above the level investigated. The relation varies with the angle of incidence of direct sunlight. The relation is not valid in all communities, and the errors involved in the assumption of its validity require further investigation.9. The leafy plant community reflects and transmits light selectively. It is important to allow for the interaction of changed spectral composition on the spectral sensitivity of the instrument when making estimates of albedo or percentage light transmission.10. Although the spectral characteristics of reflexion and transmission of individual leaves are nearly identical, this is not so for plant communities with a highly inhomo‐geneous structure.11. The percentage of total light transmitted usually increases with increasing cloud cover of the sky. The absolute intensity of diffuse light in the stand is usually higher when the sky is partly overcast than when the sky is entirely covered or clear. This is due to the increase of total light in the open with decrease of cloud cover, while the proportion of diffuse light in the total decreases with decrease in cloud cover. Provided the stand transmits more diffuse than direct light, the percentage transmission of the total will therefore fall as the total increases.12. Daily variation of diffuse light generally follows that in the open.13. In deciduous woods a light phase before leaf expansion and a shade phase after it can be distinguished. Each phase is characterized by a fairly constant percentage of diffuse light: that of direct light depends on daily and seasonal variation of solar altitude, increasing with increasing altitude.14. Light measurements given as percentages of that in the open may disguise important features of the variation of absolute amounts of light. Wherever possible these absolute quantities should be calculated. Where no direct meteorological data exist, it is often possible to make reasonable assumption from other data. The precise time, place and cloud conditions should be given for each measurement,15. Because of the great variety of instruments and techniques used, often inadequately described, comparison of results is nearly impossible. A thorough examination of the magnitude of the errors of each technique is badly

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1964.tb01164.x

出版商:Blackwell Publishing Ltd    

年代:1964

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1964.tb01165.x

出版商:Blackwell Publishing Ltd    

年代:1964

数据来源: WILEY

摘要:

Summary1. A theory has been proposed by Neuman that some metabolites which contain phosphate groups can act in the body as crystal poisons and so influence the deposition of calcium salts in skeletal structures. This theory is explained and the evidence for it reviewed.2. A crystal poison is a compound which settles on the surface of a crystal and interferes with the continued formation of the crystal lattice. Thus crystal growth stops and the mother liquor may become supersaturated.3. The evidence in favour of the theory is considered, both for phosphatic skeletons such as bone and calcareous skeletons such as those found in many invertebrates.4. Many of the phosphate groups in metabolites can produce the inhibitory effect and it has been suggested that a phosphatase enzyme acts in destroying these crystal poisons by hydrolysing them at the site of mineralization.5. Pyrophosphates are effective crystal poisons at very great dilutions, and yet they have been shown to be present in ossifying bones and in the plasma and urine of mammals.6. Alkaline phosphatase is present during the formation of calcareous skeletons, but the explanation of the removal of a crystal poison by hydrolysis is complicated by the fact that orthophosphates also inhibit calcification.7. These data are discussed in relation to the physiology of calcification in animals with phosphatic and with calcareous skeletons and some of the implications of the theory are evaluated.

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1964.tb01166.x

出版商:Blackwell Publishing Ltd    

年代:1964

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1964.tb01167.x

出版商:Blackwell Publishing Ltd    

年代:1964

数据来源: WILEY

摘要:

SummaryThe large morphological differences between plants grown in the dark and those exposed to light appear to result from an effect of light on accelerating the developmental process and causing it to proceed further. Light accelerates the early phase of expansion of stems but inhibits their final growth in length.At least two photochemical processes are involved in morphogenetic responses, the red/far‐red reversible reaction of the pigment phytochrome and a second process which only becomes important when irradiation is prolonged. This second process is particularly sensitive to blue light and frequently also to far‐red. In many cases prolonging the exposure to red light increases the magnitude of the response, even though the phytochrome equilibrium is established within minutes of the beginning of the irradiation. Nothing is known with certainty about the pigments involved in these responses to prolonged irradiation nor what kind of relationship exists between them and the reactions mediated by phytochrome. Some evidence is presented to show that the red/far‐red reaction acts earlier in the growth process than the prolonged light reaction and that, in stems, the latter acts only to inhibit elongation. There is also some indication that blue and far‐red radiation do not always act identically in a prolonged light reaction.Effective auxin levels, as measured by bioassays, are lowered after transfer to red light; the depression may be only temporary, the levels rising again later. Both co‐factors and inhibitors of IAA oxidase have been shown to be affected by light. Red light causes the production of an inhibitor of IAA oxidase. The effect of a natural inhibitor of IAA oxidase is prevented by blue light together with riboflavine. The co‐factors and inhibitors are phenolic compounds.Anthocyanin synthesis is also dependent on light and may be related to the morpho‐genetic responses. In some species both the low‐energy phytochrome reaction and a prolonged light reaction, sensitive to blue and far‐red, affect anthocyanin and growth. A number of flavonoid compounds and their precursors, especially the substituted cinnamic acids, affect the elongation of stems and roots, so that a change in the type and quantity of these compounds may be the cause of some of the light effects on growth; this may be particularly so in the case of the prolonged light inhibition of stem elongation. The compounds act as inhibitors and co‐factors of the IAA oxidase system but they may affect growth in other ways than as regulators ofin vivoauxin levels.Gibberellic acid acts by promoting stem elongation more in the light than in the dark, and it has been suggested that the light inhibition of stem elongation is caused by a lowering of the level of endogenous gibberellins. However, gibberellic acid promotes elongation whether light is promoting or inhibiting, and both the red/far‐red reversible reaction and the blue light inhibition continue to be shown in the presence of saturating doses of gibberellins. Neither the prolonged light reaction nor the phytochrome reaction, therefore, appear to lower the levels of endogenous gibberellins and the light effects are probably related only distantly to gibberellin levels. In dwarf peas, however, there is some evidence that a third photoreaction acts in reducing the effectiveness of gibberellins, perhaps by causing the synthesis of a compound interfering with gibberellin activity.The low‐energy red response probably concerns the activation of an enzyme by the formation of an isomer when a photon is absorbed; this activity is maintained in the dark for a while but the enzyme slowly reverts to an inactive form. The reactions catalysed by this enzyme are unknown, but it is probably a pacemaker enzyme acting at some key point in metabolism. The involvement of CoA metabolism has been suggested but there is as yet little evidence to support this. Phosphorylative capacity is enhanced by red light but there is no clear indication how directly this effect is related to the photochemical step. In the prolonged light reaction, enzyme activation or synthesis is probably involved. Protein synthesis and the formation of certain enzymes have been shown to require light. Nucleic acid metabolism has been shown to be concerned in some morphogenetic responses and i

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1964.tb01168.x

出版商:Blackwell Publishing Ltd    

年代:1964

数据来源: WILEY

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1964.tb01169.x

出版商:Blackwell Publishing Ltd    

年代:1964

数据来源: WILEY

摘要:

Summary1. The chemical nature of the bile salts is a character that must be under the control of several genes and is also affected by intestinal micro‐organisms and perhaps again by the liver in the course of enterohepatic circulation. Gall‐bladder bile contains the bile salts which are in use; bile from a fistula has only the primary bile salts actually produced by the liver.2. In the only invertebrate examined, the crabCancer pugurus, the bile‐salt‐like substances in the digestive juices were compounds made up of sarcosine, taurine and decanoic or 5‐dodecenoic acids.3. It has been shown for at least 13 vertebrate species (7 eutherian mammals; a bird, a boid snake, an alligator, a toad, a frog and a teleost) that the characteristic bile salts are madein viuofrom cholesterol; it is assumed that this is so in all vertebrates.4. In rats and man, 3α, 7α, 12α‐trihydroxycoprostanic acid, which contains all 27 atoms of cholesterol and is made from this compound, acts as an efficient precursor of the principal bile acid, cholic acid (3α, 7α12α‐trihydroxycholanic acid, C24H40O5). 3α, 7α, 12α‐trihydroxycoprostanic acid conjugated with taurine is a principal bile acid in all three crocodilians examined, is also present in the bile salts of two species ofRanaand has been isolated from human fistula bile. This acid is an example of substances that are intermediates in bile salt synthesis in highly evolved vertebrates and also act as bile salts in less evolved ones.5. There is clear evidence of evolution of bile salts through the stages C27(or C26) alcohols → C27acids → C24acids. Bile alcohols act as bile salts after conjugation with sulphate, C27acids are conjugated with taurine and C24acids exist in the bile as taurine and (in eutherians) also as glycine conjugates.6. Two species of hagfish (Myxinidae) have an alcohol, myxinol, as a disulphate of unknown chemical constitution.7. The coelacanthLatimeria chalumnaehas a principal bile alcohol which is the 3β epimer of cyprinol; a little cyprinol is also present. Cyprinol, 3α, 7α, 12α,26,27‐pentahydroxycholestane, is a chief bile alcohol in the dipnoanProtopterus aethiopicusand also in the five species of Cyprinidae examined. TheLatimeriaalcohol is (if not an artifact of the enterohepatic circulation) more ‘primitive’ chemically (i.e. nearer to cholesterol) than is cyprinol: it could give rise to cyprinol by evolution of a method for inversion at C‐3. Thus, all these fishes are related by the possession of a bile alcohol type not yet found in other vertebrates.8. About 34 teleostean fish species, excluding Ostariophysii, have cholic and in some cases also chenodeoxycholic (3α,7α‐dihydroxycholanic) acids, conjugated with taurine, as their chief bile salts. In some species allocholic (3α,7α,12α‐trihydroxyallo(5α)‐cholanic) acid is also present: its significance is not clear. Probably most teleosts are highly evolved in their bile salt chemistry.9. Three species of sturgeons (Acipenseridae) contain cholic and allocholic acids and small amounts of bile alcohol sulphates are also present. Identification of these may be of value in elucidating sturgeon evolutionary history.10.Chimaera monstrosahas as its chief bile salt the sulphate of chimaerol, probably 3α,7α,12α,24ξ,26‐pentahydroxycoprostane. Sharks and rays may contain a little chimaerol but the principal bile salt is the sulphate of scymnol, 3α,7α,12α,24ξ,26,27‐hexahydroxycoprostane, which could arise from chimaerol by oxidation at C‐27. The cholic acid found in selachians could be a dietary artifact; the bile salts of these fishes have either not evolved so far or have reached a different evolutionary result from those of teleosts. Scymnol is not an efficient precursor of cholic acid in the rat.11. In amphibia,Rana catesbianacontains the sulphates of 5α‐ and 5β‐ranol, i.e. 3α,7α,12α,24ξ,26‐pentahydroxy‐27‐norcholestane‐27‐sulphate and its 5β epimer; in this species 3α,7α,12α‐trihydroxycoprostanic acid was also found. InR. temporaria5α‐ranol sulphate is almost the only bile salt.R. nigromaculatahas the trihydroxy‐coprostanic acid; bile alcohols were not found. These findings put the three species in the evolutionary orderR. temporaria, R. catesbiana, R. nigromaculata. Bufo vulgaris japonicushas C27or C28bile acids with the cholic acid nucleus and also the sulphate of 3α,7α,12α,25ξ,26‐pentahydroxycoprostane. This alcohol is quite different from those found in Ranidae. Its 5α epimer has been reported in the newtDiemyctylus phyrrho‐gasterand, if this is the case, it suggests a possible link between ancestors of this animal and of Bufonidae.12. Bile alcohols have not been found in reptiles or higher vertebrates. Chelonians and platynotan lizards have (probably) 3α,7α,12α,x‐tetrahydroxycoprostanic acids that may be unique to each group; 3α,7α,12α‐trihydroxycoprostanic acid is a chief bile acid of crocodilians. C24bile acids may be general in the higher lizards and in snakes. Boid snakes have pythocholic (3α, 12α, 16α‐trihydroxycholanic) acid, formed by rehydr‐oxylation in the liver of the deoxycholic (3α,12α‐dihydroxycholanic) acid made by intestinal micro‐organisms from the primary cholic acid. 3α,7α,12α,23‐tetrahydr‐oxycholanic acid is found in some snakes as well as allocholic acid; the latter also occurs in some lizards.13. The few birds examined contained cholic, allocholic and chenodeoxycholic acids. In penguins the amount of cholic‐allocholic acid is almost the same as that of chenodeoxycholic acid, but in a few other birds examined the latter is the principal bile acid. The germ‐free domestic fowl also has allocholic acid.14. Monotremes contain cholic, chenodeoxycholic and perhaps deoxycholic acids, as do some marsupials. Glycine conjugates have not been found in these mammals or in any lower group. Koala bile salts are almost entirely taurine‐conjugated 3α‐hydroxy‐7‐oxocholanic acid.15. Eutherian mammals usually have cholic and chenodeoxycholic as primary bile acids. Herbivores (except bovids) often have a preponderance of dihydroxy acids, frequently as glycine conjugates; omnivores have a mixture of tri‐ and dihydroxy acids as glycine and taurine conjugates and carnivores have taurine‐conjugated trihydroxy acids. Glycine conjugation in some species is apparently less well established than taurine conjugation; but dietary deficiencies can increase glycine conjugates. Unique bile acids, certainly or probably primary, have been found in Murinae (3α,6β,7α‐ and 3α,6β,7β‐trihydroxycholanic acids),Sus(3α,6α,7α‐trihydroxycholanic acid) and all Pinnipedia (3α,7α,23‐trihydroxycholanic acid). Other substances, such as ursode‐oxycholic (3α,7β‐dihydroxycholanic) acid, may be wholly or partly artifacts of the enterohepatic circulation; they may nevertheless be physiologically important. Deoxycholic acid (as its glycine conjugate) is normally the chief bile acid in rabbits, although it is an artifact, but Murinae re‐hydroxylate it to cholic acid. The biochemical status of 3α‐hydroxy‐7‐oxocholanic acid is disputed.16. Animals with primitive bile salts often also contain small amounts of more evolved types; the beginnings of bile salt evolution can be detected long before it appears likely to affect the physiological behaviour of the bile salts as a whole.17. The physiological functions of the bile are not sufficiently understood to permit of speculation about the advantages of any particular type of bile salt.18. Biochemical studies show that there are even more interspecific differences between bile salts than the chemistry alone suggests. Such essentially enzymic studies approach an understanding of the genes controlling the chemical characters reviewed here and may eventually throw light on fundamental q

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1964.tb01170.x

出版商:Blackwell Publishing Ltd    

年代:1964

数据来源: WILEY