The objective of this study was the evaluation of herbage production and botanical composition of a native grassland, ("flechilla?'), of the Salado River Basin fertilized, during 3 years, with 0, 381, or 762 kg&a-'*yr-' of ammonium sulphate alone or combined with 0 or 208 kg-ha-'*yr-' of triple superphosphate. Annual dry matter accumulation of herbage, (ADMA), was restricted by a negative water-balance in the soil during the first experimental year. A linear response to ammonium sulphate rate, irrespective of triple superphosphate, was observed. In the following years the relationship between ADMA and ammonium sulphate became progressively quadratic, and it was manifest firstly in the plots fertilized with superphosphate. The response to superphosphate seemed to be due to an environmentally stimulated growth demand more than to a phosphorus deficiency in the soil. Ammonium sulphate promoted the growth of gramhtoids and decreased that of legumes and forbs. Superphosphate increased the proportion of legumes and ameliorated the detrimental effect of ammonium sulphate, but only few species reflected the effect of fertilization. Because (a) the sward reacted remarkably to the addition of a nitrogenous fertilizer, and (b) its main legume species, Medicogo polymorpha, (annual), was scarce, it is suggested that the sward's herbage production could be substantially increased by its enrichment with perennial legumes provided that their growth and expansion were assisted by periodic additions of a phosphorous fertilizer. Authors are research scientists, Centro de Ecofisiologia Vegetal (FECIC-CONICET-Fund. M. LILLO), Serrano 661, 1414 Capital Federal, R. Argentina. Authors wish to acknowledge Dr. Ratil Arraras Vergara for having permitted the conduction of the present trial in his farm "El Tr.&ito"and Ings. Agrs. E. Myers, R. Mendoza, a@ Mariana Kade for their invaluable help in the collection of data. Manuscript received April 18, 1980. JOURNAL OF RANGE MANAGEMENT 35(l). January 1982 Thurston, J.M. 1969. The effect of liming and fertilizers on the botanical composition of permanent grassland, and on yield of hay. In: T.H. Rorison (ed.), Ecological Aspects of the Mineral Nutrition of Plants, p, 3-10. Blackwell Scientific Publications, Oxford. Vervoorst, F. 1967. "La Vegetaci6n de la Repiblica Argentina. 7. Las Communidades Vegetales de la Depresi6n de1 Salado. (Pcia. de Buenos Soils". 6th Edition. MacMillan, New York. 567 p. Aires)". Serie Fitogeogritica. INTA. Buenos Aires. 259 p.

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There is a wide variation in the nuclear DNA content and chromosome size between the species belonging to the T. crassifolia and T. virginiana alliances (all the species but one are native to Central and North America). Also the DNA content per genome decreases when the ploidy level increases within the same specific polyploid complex with three ploidy levels (2x, 4x, and 6x). In contrast, no variation was found in the DNA content per genome between different ploidy levels in the T. fluminensis alliance (all the species are native to South America) where they range from 6x to 22x. Since all the species described here are perennials with various life forms, it was possible to analyze the relationship between the DNA content and their vegetative adaptation to the environment. The more specialized species (geophytes and hemicryptophytes) have a higher amount of DNA than the chamaephytes adapted to live in relatively more mesic regions. In the species living in Central and North America there is a positive correlation between the increase in DNA content and the latitude of their native regions.Key words: Tradescantia, DNA content, geographical distribution, life forms, polyploidy.

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Abstract. Tendrils are thread-like organs whose function is to support the stems of many species of climbing plants. Tendrils naturally move (circumnutate) in space. Individual tendrils of cucumber (Cucumis sativus L.) 'Poinset' had the vertical component of their mean velocity vector of circumnutation changed when exposed to a range of weak static magnetic fields between 1 and 16 mT. The speed (modulus) of the velocity vector was significantly increased (p = 0.016) in the vicinity of a magnet, but its direction did not show a definite trend with respect to the magnet. Although cucumber tendrils bear static positive charges, they did not behave as charged bodies do in a magnetic field, neither did they show a magnetotropic response. In fact, tendrils showed a nastic response to magnetism. Magnetic fields affected some processes underlying the movement of circumnutation, but no clear interpretation of them can be given presently on the basis of the known effects of magnetism on plants. It is clear that cucumber tendrils, because of some inherent susceptibility to magnetism or their particular size and shape, are very sensitive to relatively low static magnetic field strengths.

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