A new species of Ceratozamia (Zamiaceae) from Chiapas, Mexico

Ceratozamia zoquorum sp. nov. from the northern mountains of Chiapas, Mexico, is described and illustrated. It has affinities with C. miqueliana Wendl. from Veracruz, but differs in leaf, male female cone and trunk morphology.     

The Response of Primordial Cells of Vicia faba to Colchicine

Whole root systems of Vicia faba were treated with 0.025 percent colchicine for 3 h. Meristems of roots and developing primordiawere examined immediately after treatment and 24 and 48 h later.Large primordia, i.e. those primordia that would emerge as lateralroots within approximately 24 h, are induced to form tetraploidcells. However, unlike lateral roots, they do not form a typicalc-tumour, their longitudinal growth is not inhibited, and theirmitotic index does not fall. Furthermore, the tetraploid cellsinduced in large primordia are not maintained when the primordiaemerge as lateral roots. Since the response of the very largestprimordia to a 3-h treatment with 0.025 per cent colchicineis similar in every way to that of lateral roots, it appearsthat the cells of primordia acquire the characteristics of lateralroot meristematic cells before the primordia emerge. From theevidence that induced tetraploid cells fail to survive as aprimordium grows out to form a lateral, it also seems that astructural reorganization accompanies the physiological reorganizationthat occurs as a primordium emerges.     

Effect of Nitrogen Supply on the Grass and Clover Components of Simulated Mixed Swards Grown under Favourable Environmental Conditions II. Nitrogen Fixation and Nitrate Uptake

Simulated mixed swards of Perennial Ryegrass (Lolium perenneL.) cv. S23 and White clover (Trifolium repens L.) cv. S100were grown from seed under a constant 20 °C day/15 °Cnight temperature regime and harvested at intervals over and88 d growht period. The swards received a nutrient solutiondaily, which was either High (220 mg l^–1) or Low (10 mgl^–1) in nitrate N. The nitrate was labelled with the ¹⁵Nisotope. An acetylene reduction assay was carried out on eachsward just prior to harvest. Rates of acetylene reduction agreed qualitatively with the ^l5Nanalyses but absolute values did not match (assuming a 4:1 C₂H₄:N₂ratio) and errors in the acetylene assay are discussed. In theLow-N swards clover relied almost entirely on symbioticallyfixed N₂, fixing more than ten times as much as the High-N cloverplants. In the Low-N treatment the grass was N-deficient despiteobtaining much more nitrate per unit root dry weight than clover.In the High-N swards, however, clover took up more nitrate perunit root weight than grass. The High-N clover plants also fixedsome N₂ and maintained a higher total-N content than grass throughoutthe period. There was no evidence of transfer of symbioticallyfixed N from the clover to the grass in either treatment. Trifolium repens, Lolium perenne, nitrate, nitrogen fixation, ¹⁵N, acetylene reduction     

The Response of Atriplex amnicola to the Interactive Effects of Salinity and Hypoxia

The growth of Atriplex amnicola, its water and ion relations,and carbohydrate use were investigated in response to the interactiveeffects of salinity and root zone hypoxia in an experiment conductedin nutrient culture. One week of hypoxia in the root zone atboth 50 and 400 mol m^–3 NaCl caused the cessation of rootgrowth, a reduction in shoot growth, and adversely affectedwater relations, but not ion relations or carbohydrate concentrations.Two weeks of hypoxia at 400 mol m^–3 NaCl resulted in thedeath of root tips, a 20–fold increase in the resistanceto water flow from the exterior of the roots to the leaves,and a further deterioration in water relations. There was alsoa doubling of Cl^– concentrations in the xylem sap anda doubling of Na⁺ and Cl^– concentrations in the leaves.An increase in the concentration of starch in the leaves, andsugars in the leaves, stems and roots, indicated that therewere problems with carbohydrate use rather than supply. Underthe prevailing conditions of low vapour pressure deficit, iontoxicity was the most probable cause of injury to A. amnicolain hypoxic solutions at high salinity. The response of A. amnicolato the interactive effects of salinity and hypoxia were similarto those reported for non-halophytes, but occurred at highersalinities. Key words: Atriplex, hypoxia, salinity, water relations, ion transport, carbohydrate     

The Gastropod, Siphonaria pectinata: a Factor in Destruction of Beach Rock

The marine gastropod, Siphonaria pectinata, has an active partin the complex formation of secondary solution features characteristicof outcroppings of beach rock in south Florida. Analyses weremade of the distribution, anatomy, and feeding activity ofthissnail. The results show that these molluscs are concentratedin areas of maximum algal growth. While grazing they rasp substratapresoftened by algae so that new rock surfaces are continuallyexposed to biochemical erosion.