Nervous control of mammalian salivary glands

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Abstract
Tree foliage has to withstand many environmental - especially climatic - stresses, but does not show symptoms of stress readily. Nevertheless it may react invisibly. This paper presents evidence that trees are particularly prone to SO$_2$ effects before or without the appearance of visible symptoms of injury. Of the various factors that affect tree sensitivity to SO$_2$ only six are mentioned: increased wind velocity in the huge crowns, longevity of foliage, sulphur uptake, supply of nutrients and water, genetic variability and pollution regime. Biological indications such as direct effects of SO$_2$ on trees may be expressed either quantitatively or qualitatively. The economic point of view often considers only the quantitative aspect: the immediate effect on growth or wood production. This depends on CO$_2$ uptake, which in turn is influenced by SO$_2$. Growth - for example ring width or root weight - is affected in consequence. There may also be effects on buds and generative organs, increased susceptibility to abiotic (snow, frost, etc.) or biotic factors (insects, fungi, etc.), and metabolic changes. The last include increased need for detoxification, changes in enzyme activity or in amounts of organic compounds and increased membrane permeability; all are expressions of an increased risk.