Distribution and feeding of larval Chironomidae in a gravel-pit lake

SUMMARY. Replicated benthic samples were collected from a wet gravel-pit in the English Midlands at fortnightly intervals during 1978. The aggregation patterns of the four most numerous genera, Chironomus, Polypedilum, Procladius and Tanypus , were analysed by fitting the means and variances of replicated sampling units to Taylor's power law and Iwao's plot of mean crowding against mean density. Chironomus larvae were not aggregated; all other genera showed an increased aggregation with density. The Iwao plots indicated that Tanypus larvae tended to be aggregated at low as well as high densities. Gut contents were analysed in larvae of the above genera and also in Cryptochironomus larvae, to investigate the relationship between dispersion pattern and feeding. Chironomus and Polypedilum larvae fed mainly on detritus, Tanypus on algae, Procladius on algae and crustacea and Cryptochironomus on oligochaetes. The extent of overlap in feeding niche was measured by Levins' index and shown to be low in all except the pair feeding upon detritus. Differences in the feeding niche may partly account for the dispersion patterns of the genera investigated.     

On the structure and evolutionary relationships of the Cerro Cuadrado fossil conifer seedlings*

Seedlings from the Cerro Cuadrado (Jurassic) Petrified Forest in Argentina have been shown to be comparable to extant araucarian seedlings from the sections Eutacta, Bunya and Colombea. Anatomically they are compared to the cone axes and embryos in the fossil cones of Araucaria mirabilis and Pararaucaria patagonica. Two seedlings previously described by Wieland are shown on pith structure to belong to P. patagonica. The remaining turbinate to top-shaped structures are compared with first year extant Bunya and Columbia seedlings and the cone A. mirabilis. Corm-like structures may represent an older seedling stage. From earlier work on the embryonic anatomy and germination stages described here, P. patagonica is deduced to have been epigeal and A. mirabilis hypogeal.     

The First Appearance of Polyploid Nuclei in Primary Roots of Two Diploid Angiosperms

In Anchusa capensis, where the polyploid divisions are restrictedto a particular time and place in germination, cytophotometricanalysis showed that the polyploid cells arise at the time ofconversion of the globular-shaped embryo to the heart-shapedembryo. In Spinacia oleracea, where the divisions occur throughoutthe growth of the root, these polyploid cells do not arise duringembryogenesis but during germination and subsequent growth. Anchusa capensis Thunb., summer forget-me-not, Spinacia oleracea L., spinach, root meristem, polyploid divisions, determinate divisions, indeterminate divisions, germination, embryogenesis, cytophotometry