The Development of a Quiescent Centre in Lateral Roots of Vicia faba L.

During lateral root elongation there was a reduction in thenumber of cells present in the apical mm of the root, in thewidth of the root and in the number of rows of cells acrossthe root. At the same time, a quiescent centre, which was absentfrom large primordia and from lateral roots which had just emergedfrom the tissues of the primary root, was established. The quiescentcentre increased in size and cell number as the lateral rootelongated from 0·2 to 4 cm, the most rapid increase takingplace in roots between 0·2 and 0·5 cm in length.There was no correlation between root width and size of thequiescent centre in these roots. The cells in the quiescentcentre were smaller in size and had a lower labelling indexthan cells in other parts of the meristem. The distributionof labelled nuclei within the quiescent centrewas determinedand is discussed with respect to the site of root initial cells.     

Sediment development and its influence on the distribution and tube structure of Chironomus plumosus L. (Chironomidae, Diptera) in a new impoundment

Field and laboratory evidence is presented to demonstrate that colonization of a new impoundment by the early invader, Chronomus plumosus L., is strongly influenced by sediment, the depth of which affects both larval abundance and tube shape. Larvae avoid areas with little sediment, such as eroded or newly inundated shores. In laboratory experiments, larvae constructed horizontal tubes in sediments between 3 and 10 mm depth, but in deeper sediments (11–30 mm) most of the larval tubes were vertically orientated. Of these about 70% were U-shaped and the remainder J-shaped. Larval abundance was positively correlated with sediment depth in the range 0–10 mm.     

Refugia and habitat partitioning among midges (Diptera: Chironomidae) in rain-pools

ABSTRACT.

• 1 Rain-pools on granite exposures are common in tropical Africa and they are inhabited by the larvae of two midge species, Chironomus imicola Kieffer and Chironomus pulcher Wiedemann.
• 2 These pools differ from other ephemeral island habitats such as carrion, fallen fruit, sap flows, mushrooms and dung in their high degree of spatial predictability. Most pools are predictably inhabited by either C.imicola or C.pulcher. This finding is considered in the light of contemporary competition theory.
• 3 An arrangement whereby the same species repeatedly invades the same temporary habitat indicates that there is something about the habitat itself causing the discrimination. I suggest that degree of exposure to sunlight may provide a cue allowing females to discriminate between pools as oviposition sites.
• 4 Both species survive the long dry season by retreating to pools of river water left by receding rivers. Dispersal to rain-pools by emerging adults occurs during the following season with imicola occupying rain-pools remote from shaded rivers. Appropriately, measured against pulcher, the biology of imicola is that of a colonizing species.
• 5 The possibility is discussed that the chironomids in rock-pools do not survive to breed. Instead, these populations may be maintained externally, for example by invasions from breeding populations in the Great Lakes of Africa, where larval densities may be too low to be detected by sampling. The evolutionary implications of such a situation are considered.

     

Structural Similarities between α-Lytic Protease of Myxobacter 495 and Elastase

A model based on the structures of α-chymotrypsin and elastase suggests an explanation for the close resemblance in the catalytic behaviour of these two enzymes.     

An alternative mating system in small male insects

Abstract. 1. Small males of all midge (Diptera: Chironomidae and Chaoboridae) species thus far examined (one chaoborid and six chironomids) are rare in mating swarms. They are found instead in vegetation adjacent to the swarm.
2. We show that it is here that females aggregate prior to embarking on mate acquisition flights. In the vegetation females appear to be accessible to males staying behind.
3. Such behaviour in small males may exploit the mate-attracting activities of large males in the swarm and may, at the same time, reduce competition and conserve energy.
4. The evolution and maintenance of these size-related types of mating behaviour are discussed.     

Habitat distribution and body size in rain-pool dwellers

Rain-pools differ from oceanic islands, essentially, in being ephemeral. Ability to disperse and reproduce is therefore a special priority among inhabitants.
Pupal exuviae of a principal inhabitant, Chironomus imicola , were collected from pools in tropical Africa. Length of pupal skin is positively related both to flight duration and ovule number in adult chironomids. Hence length of pupal skin can be related to fitness in this temporary pool dweller.
Predictably, therefore, remote pools are inhabited by large individuals while those in clusters have more small forms.
That duration of the habitat also influences body size is demonstrated by Polypedilum vanderplanki. These larvae are able to survive dry periods and therefore experience the habitat as permanent. In contrast to C. imicola, P. vanderplanki responds to isolation with a disproportionate number of small flies.
In addition to these differences in average size, variance in body size is affected by isolation of the habitat. Comparison with progeny of a single female shows increased variability to be determined not only by habitat location but also to be an inherent characteristic of animals perceiving the habitats as ephemeral.     

DEVELOPMENT OF SOME LAKE ECOSYSTEMS IN TROPICAL AFRICA, WITH SPECIAL REFERENCE TO THE INVERTEBRATES

1. The flooding of a lake basin initiates a series of changes leading eventually to a more stable climax situation after some years. Sequential physical and chemical changes in the mud and water and related changes in the animal and plant populations of three types of tropical African lakes are considered. The giant man-made lakes, Kariba and Volta, both several thousand square kilometres in area, provide the bulk of the material for this review. Two other kinds of tropical lake, the annual storage-reservoirs like Jebel Aulia and Sennar in the Sudan, and natural lakes subject to periodic droughts, like Lake Chilwa in Central Africa, are also considered. Evidence is often patchy but suggests a number of generalizations regarding the course and causes of the developmental changes in these tropical ecosystems. 2. On the evidence available, the course of development appears to fall conveniently into two periods based on water level changes, the filling phase and the post-filling phase. The former is characterized by sudden appearances of organisms and explosive growths of animal and plant populations. This unstable behaviour, especially characteristic of tropical lakes, is associated with the destruction of old habitats and the creation of new ones, the increasing area and volume of the new lake environment and the introduction of materials to the system at the advancing shore-line. By the time that filling is complete, the situation has stabilized to a large extent and the ecosystem enters a new phase. 3. This post-filling phase is characterized by the development and exploitation of existing habitats. Examples considered here are: the development of the mud habitat under the influence of processes like sedimentation and beach formation; the development of submerged woodland as a habitat for bottom dwelling animals; the spread of rooted aquatic plants and their effect on the mud; and the role of fluctuation in water-level on the post-filling phase. All four phenomena result in a modification of the substrata originally flooded, in a way that directly effects the associated fauna and flora. Apart from influencing the first three processes, water-level fluctuations also result in an interaction between the aquatic and terrestrial ecosystems which brings about important changes in itself. 4. The relative importance of these two phases varies with the type of lake, filling-phase phenomena obviously dominating in the annual storage reservoirs while post-filling phase characteristics are fully expressed in the large lakes made by man. 5. In contrast to the development of temperate lake ecosystems, succession of species in these tropical examples is not so much interrupted by major annual temperature changes. In addition, both the course of succession and the climax communities achieved are different. This may be largely due to the more rapid decomposition of organic matter in the muds which has two main consequences. In the first place, extraneous organic material brought in to the system during flooding, is rapidly broken down. Early filling stages, therefore, are associated with anoxic conditions and development of conspicuous communities of algae and large aquatic plants. The result is an overlapping of what in temperate systems are two distinct episodes: the early extraneous and later self-contained systems. Secondly, the replacement of chironomids by an oligochaete climax in the mud as seen in temperate lakes does not normally occur, apparently because of the lack of accumulated organic material. The presence in Africa of the may-fly Povilla adusta, on the other hand, provides an extra stage in the colonization of submerged woodland, replacing the earlier chironomid communities.     

Development of the mud habitat during the filling of two new lakes

An attempt is made to illustrate some of the principles of early development of muds in two contrasting lake basins which were devoid of vegetation cover before filling. Essentially the changes involve the dispersion and redistribution of particles in the newly flooded lake bottom. Field and laboratory observations are presented to demonstrate the interaction that exists between water chemistry, wave erosion and the burrowing activity of mud-dwelling insect larvae, and the effect of these factors on particle size sorting in the substrate. These characteristic changes in the mud during filling are compared with post-filling stage developments in which the role of extraneous material, including organic matter, may predominate.