Science as a Way of Knowing--Human Ecology

This essay is part of the second presentation of an educationalproject of the American Society of Zoologists. The purpose ofthe project is to offer suggestions for improving the first-yearbiology courses in the universities. The method consists ofemphasizing the conceptual framework of the biological sciences,showing how scientific information is obtained and evaluated,pointing out the strengths and limitations of scientific procedures,and above all showing the relevance of science for human hopesand welfare. Each year a major topic will be considered. Lastyear it was Evolutionary Biology. This year it is Human Ecology.     

The Effects of Temperature on Root Initiation in Detached Cotyledons of Sinapis alba L.

A thermoplate system is described which enables uniformly illuminatedpetri dish cultures to be maintained under a range of temperatureregimes. The system is evaluated and used to investigate theeffects of incubation temperatures between 10 and 35 ?C on rootinitiation. Sinapis cotyledons root most rapidly and in greatest numbersat 25 and 30 ?C. Below 25 ?C, rooting is reduced and delayed,and above 30 ?C rooting is prevented. The optimum temperaturefor initiation of primordia is close to 25 ?C, where, after4 days over 80 per cent of petioles have at least one primordiumand over half of these are already elongating. Initiation isconsiderably reduced at temperatures above 30 ?C and below 25?C and primordium elongation either fails to take place (35?C) or is delayed (20, 13, 10 ?C). At 25 ?C, 90 per cent of the primordia are initiated in theterminal 1.5 mm of the petriole. A similar pattern occurs atother temperatures, but the relative numbers of primordia initiatedin the upper petiole are increased when cotyledons are incubatedat temperatures below 25 ?C. At 25 ?C a higher proportion ofprimordia are initiated in the upper petiole in cotyledons withmany primordia than in cotyledons with few, but the developmentof primordia to the elongation stage is not affected by theirposition in the petiole nor by the numbers of primordia present.     

Chemotaxonomy, variation and geographical distribution of the Empetraceae

Thirty-one flavonoids (and related phenolics) were found in leaves of the six species in the three genera of Empetraceae (90 populations studied) and five further pigments in fruits of Empetrum. Affinity with the Ericaceae is supported by the occurrence of the 3-galactosidesof gossypetin, malvidin, delphinidin, cyanidin and peonidin. The distribution of leaf flavonoids agrees with both the recognition of three genera and species-delimitation in Ceratiola and Corema. In Empetrum , the flavonoids differ modally in the three species usually accepted, thus confirming data on fruit colour and breeding systems, and suggesting that the genus comprises one species with about three subspecies at diploid and tetraploid levels. The black fruits of most Northern Hemisphere plants contain malvidin, petunidin and delphinidin, while the red fruits of almost all Southern Hemisphere populations ( E. rubrum ) contain cyanidin and peonidin and provide a rare example of the association of cyanidin (instead of delphinidin) with the wild-type allele.
In leaf flavonoid content the diploid E. nigrum is much closer to its presumed tetraploid derivative E. eamesii than to the diploid E. rubrum , which differs most from E. eamesii. Flavonoid patterns and breeding system distinguish most E. rubrum populations from those isolated in Tristan da Cunha-Gough Island. The phytogeographical importance of the phytOGhemical results is discussed.     

Acid-Efflux Patterns of Primary and Secondary Roots of Ricinus communis (Euphorbiaceae)

Graviresponding primary roots of castor bean (Ricinus communis)were characterized by more acid efflux on their upper (i.e.rapidly growing) sides than their lower sides. Acid-efflux patternsof the upper and lower sides of horizontally oriented lateralroots were symmetrical. The onset of graviresponsiveness bysecondary roots correlated positively with the development ofasymmetric patterns of acid efflux similar to those of gravirespondingprimary roots. The addition of 1 mM sodium orthovanadate (aninhibitor of auxin-induced proton efflux) to the growth mediumabolished gravicurvature and the development of acid-effluxasymmetry by primary and secondary roots. These results suggestthat (i) the absence of an auxin-induced asymmetry of acid effluxmay be the factor responsible for uncoupling graviperceptionfrom gravitropism in lateral roots, and (ii) the developmentof an auxin-induced asymmetry of acid efflux may mediate theonset of graviresponsiveness by secondary roots of R. communis. Ricinus communis, castor bean, acid efflux, root gravitropism     

Changes in Leaf Composition in Parthenocissus tricuspidata Planch. During Growth and Senescence of Short Shoots

During expansion of the pre-formed leaves the content of sodium,potassium, chlorophyll and anthocyanin increased for a time,fell, then increased again at the end of the period of leafgrowth. The final increase in anthocyanin content was relatedto the formation of a red margin around the leaves, which wasaccompanied by a loss of chlorophyll in the margin tissue. Thiswas not related to the fall in total leaf chlorophyll whichoccurred earlier. The temporary loss of sodium and potassium from the leaves wascorrelated with the development of axillary inflorescences andan increased rate of shoot growth. Subsequent recovery of leafsodium and potassium took place as the shoots senesced. Shootsfrom the centre of the vine grew slowly and died back to thesecond node, forming spur shoots, whereas at the vine margin,shoots grew more rapidly and died back less.     

Function of leaf hamamelitol as a compatible solute during water stress treatment of Hedera helix L.

Hamamelitol is an unusual branched-chain sugar alcohol previously suggested to function as a leaf compatible solute. In this study, we have examined the leaf metabolism and intracelluiar compartmentalization of hamamelitol and other soluble sugars during long-term water stress treatment of Hedera helix (English ivy). Total leaf hamamelitol content was relatively low in greenhouse control plants, but increased 2-fold during water stress treatment to levels approaching those observed in field-grown plants (6–7 μmol g^?1 fresh weight). Using density gradient fractionation with non-aqueous solvents, we showed that hamamelitol occurs primarily in the cytoplasm and vacuoles of leaf mesophyll cells. During water stress treatment most of the increase in leaf hamamelitol occurred in the mesophyll cytoplasm, compensating osmotically for a decrease in cytoplasmic sucrose concentration. The maximum concentration of cytoplasmic hamamelitol was 155 mol m^?3 and occurred in field-grown plants. Labelling experiments showed that hamamelitol is slowly synthesized from ¹⁴CO₂ in leaves of H. helix, but is very long-lived (estimated t_1/2 of 4 years). Together, these data indicate that hamamelitol probably functions during long-term stress conditions as an osmotically active, compatible solute in plant leaves. We suggest that the signal for enhanced accumulation of hamamelitol during the water stress treatment was initiated by decreased plant growth and increased leaf sucrose hydrolysis.