摘要:

Summary(i) The tropical rain forest is a formation‐type or pan‐climax. Its outstanding characteristic is the enormous wealth of species of which it is composed. (2) The investigation of the floristic composition of the rain forest presents special difficulties owing to the inaccessibility of the tree‐tops, non‐seasonal flowering and the backward state of taxonomy in tropical regions. For these reasons little definite information has been forthcoming on the subject until recently and the rain forest in each of the main geographical regions has been regarded as forming a vast assemblage of species varying in composition from place to place and not differentiated into distinct forest types comparable to the associations and consociations of temperate forests. (3) The floristic composition can best be studied by counting the trees on sample plots, identifying them first of all by their vernacular names with the help of native ‘tree‐finders’. With sufficient precautions this method can be made to give results of scientific value. (4) Primary lowland rain forest may be either Mixed Forest with numerous dominants or one dominant may form a large proportion of the stand. (5) Mixed rain forests may contain up to 100 species of trees 8 in. diameter and over in a sample plot 400times400 ft. The majority of the species in such a plot are represented by one or very few individuals, the average number of individuals per species (Mischungs‐quotient)for trees 8 in. diameter and over being sometimes as low as 2–7. The evidence suggests that in any one geographical area there is only one mixed association whose composition fluctuates in both space and time. (6) Forests in which one species forms a large proportion of the whole stand are found in all the major tropical regions, but occupy small areas compared with the mixed associations. Some of these single‐dominant types are comparable with the consociations of temperate forest climaxes, while others should probably be regarded as societies of a gregarious species. In South America and Africa the majority of the dominants of consociations belong to the Leguminosae. (7) Most, perhaps all, of these single‐dominant communities occupy special soils or habitats and it is suggested that they tend to be restricted to non‐optimal soils while the mixed association occupies optimum soils. On similar soils in geographically widely separated areas there tends to be a parallel differentiation of edaphic forest types. (8) The single‐dominant forests can probably be regarded as what Braun has termed ‘association‐segregates’. If this is so the tropical rain forest, like the ‘Mixed Mesophytic Forest’ of North America, would consist of undifferentiated climax associations together with association segregates which are consociations, i.e. associations with a tendency to single‐species dominance.I would like to express my gratitude to Mr T. A. W. Davis and Mr T. G. Tutin for communicating unpublished data, and to Dr H. Godwin and Mr T. G. Tutin for kindly r

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1945.tb00310.x

出版商:Blackwell Publishing Ltd    

年代:1945

数据来源: WILEY

摘要:

Summary(i) When nuclei are irradiated by X‐rays, neutrons or radioactive radiations either clumping of the chromosomes or breaks and structural rearrangements of the chromosomes are observed at metaphase according to the division stage, respectively late prophase and metaphase or resting stage and early prophase, reached at the time of radiation. (2) Chromosome structural changes are induced at the resting stage, chromatid at early prophase. The number of primary breaks induced greatly exceeds the total number of breaks observed at the subsequent metaphase. The majority of the breaks restitute after being open for a few minutes. Exchanges are possible during the time the breaks are open. If the dose of radiation is given at a high intensity, in a short time, so that all the primary breaks coexist, the yield of X‐ray induced exchanges is proportional to the square of the dose. If the dose is given at a lower intensity, in a longer time, some restitution of the earlier formed breaks occurs before the later ones are formed, and the yield of exchanges is then proportional to a lower power of the dose. (3) Only breaks with an initial separation of not more than 1 μ have an appreciable chance of taking part in an exchange. (4) In neutron and α‐ray experiments, in which a small number of ionizing particles cross the nucleus, the yield of exchanges is linearly proportional to dose and independent of intensity. Thus the majority of the exchanges are between pairs of breaks produced simultaneously by the same ionizing particle. (5) An ionizing particle may break both chromatids of a split chromosome at the same locus and an isochromatid break usually results. (6) A proportion of the primary chromatid breaks are unjoinable; hence a proportion of interchanges are incomplete, a proportion of isochromatid breaks show non‐union and a proportion of breaks not taking part in exchanges persist as visible chromatid breaks instead of restituting. (7) The number of primary chromatid breaks can be inferred (Table 2) and are found to differ for different radiations. A proton or an α‐ray traversing a chromatid has almost unit probability of causing a break. An electron is likely to cause a break only if the last 0–3 μ of its ionized track traverses the chromatid; hence a minimum of 15–20 ionizations must be produced in a chromatid of diameter 01 μ for the breakage probability to approach unity. The relative efficiencies of different wave‐lengths and types of radiation can be explained on this basis.I am greatly indebted to Dr D. E. Lea, Strangeways Laboratory, Cambridge, for his collaboration on radiation problems and especially on the present occasion for most generous permission to use unpublished calculations without which the present account would have been

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1945.tb00311.x

出版商:Blackwell Publishing Ltd    

年代:1945

数据来源: WILEY

摘要:

Summary(i) The general disposition of the sensory, association (internuncial) and motor elements of the starfish arm as revealed by intra‐vitam staining is described and figured (Fig. i), and (2) the nature of the nervous arcs responsible for reflex activities is discussed. (3) It is pointed out that the nature of the activities of an organ and the degree of integration of groups of organs are conditioned by the form and properties of the neurones and tracts through which they are innervated. (4) The pedicellariae, papulae and spines which are situated on, or are part of the dorsal body wall of a starfish are innervated through a plexus which has peripheral qualities. (5) There is little evidence of anatomical differentiation within the peripheral plexus except for the presence of groups of transversely running tracts which may correspond to the through‐conduction tracts (vide infra(8)). (6) The properties of the peripheral plexus are those of a nerve net as defined by Pantin (1935a, c) and the movements of the pedicellariae are analysed in relation to these properties. (7) The net shows evidence of differential capacity for facilitation in its various parts, in particular (8), there are transverse through‐conduction tracts in the plexus which, when coupled with the lateral motor systems of neurones (Eof Fig. i) are the means of innervating the papulae and muscles of the body wall. (9) The movements and postures of the tube feet are described. (10) Together with the capacity of the feet to be co‐ordinated throughout the entire animal they demonstrate the central nervous properties of the circumoral ring and radial cords through which they are innervated. (11) These central nervous properties derive from the presence of special centres of co‐ordination, from a marked polarity of conduction, and from a certain configuration of the motor neurones within the central nervous system and the tube feet. (12) An account is given of the probable location and functions of the nerve centres and of the disposition of the nerve tracts within the central nervous system. (13) A brief reference is made to the effects of peripheral excitation on the pattern of coordinated movement of

ISSN:1464-7931    

DOI:10.1111/j.1469-185X.1945.tb00312.x

出版商:Blackwell Publishing Ltd    

年代:1945

数据来源: WILEY