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In this thesis is described the characterisation of two morphogenesis mutants of Arabidopsis thaliana, with the aim of furthering our understanding of signalling in development. A thorough phenotypic analysis of both mutants is presented, with particular attention paid to the seedling root. The results of genetic analysis of hydra2 are also presented. The hydra mutants are characterised by a pleiotropic phenotype, with defective embryonic and seedling cell patterning, morphogenesis and root growth. The HYDRA1 gene encodes a A8-A7 sterol isomerase, while HYDRA2 encodes a sterol C14 reductase, previously identified as the FACKEL gene product. Seedlings mutant for each gene are similarly defective in the concentrations of the three major Arabidopsis sterols. Reporter gene analysis showed mis-expression of the hormonally-regulated DR5, IAA2 and ACSl promoters and of the epidermal cell file-specific GL2 promoter in the mutants. The mutants also exhibit enhanced responses to auxin. The phenotypes can be partially rescued by inhibition of auxin and ethylene signalling, but not by exogenous brassinosteroids. Evidence of abnormal activity of hormone-related membrane-bounds proteins and of altered membrane permeability to auxin is presented. A model in which correct sterol profiles are required for regulated auxin and ethylene signalling through effects on membrane function is proposed.

The ontogenetic development of the secondary sieve elements in ash, Fraxinus excelsior is traced by using mainly conventional electron microscopy. In addition, negative staining of the exudate and light microscopy have also been used. No major differences from the established trends have been observed in the differentiation of these sieve elements. There is evidence that the P-protein in the sieve elements originates from helical polysomes found scattered in the dense cytoplasm of young sieve elements. These polysomes also give the appearance of having been derived from rough ER cisternae. Reorganisation of the fibrillar P-protein into the tubular form also seems to take place in the larger P-protein bodies. In negatively-stained preparations, a suggestion of a tightening of the double helix of the P-protein fibrils is discernible. This reorganisation is however not consistent. The usual banding of the fibrillar P-protein is prominent. Most of the mature sieve plates show occluded pores either callosed or not. The P-protein frequently appears to fray out on the downstream side and sometimes on the upstream side as well. This has a bearing on the nature of functioning sieve plates and does not seem to be due to artifacts. Empty spaces which represent probably a peculiar artifact are observed immediately above most of the mature sieve plates. P-protein seems to pile above these "barrier" spaces while the sieve plates remain occluded with P-protein. Other formations of peculiar interest are the plasmalemmal invaginations ("blebs") found both in the sieve elements and in the parenchyma cells. In the sieve elements, their formation seems to be in an endocytic (pinocytotic) direction. In terms of the sieve tubes, these "blebs" have a possible significance in the uptake of solutes and ions.

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Variation in structure of the secondary xylem was studied in the hole, branches and roots of Quercus robur L., Fagus sylvatica L., Acer pseudoplatanus L., and Robinia pseudoacacia L. Attention was paid especially to variation in the length of the vertical elements, diameters of the vessels and to ray height and volume. Vessel-elements and libriform fibres increased in length from the pith outwards at any one level in the bole, branch and root, at first rapidly and then rather slowly. In a specific growth ring these elements increased in length upwards in the bole up to a certain height and later decreased. It is suggested that the fusiform initials of the vascular cambium become larger during the first few years when growth of the tree is most vigorous, thus accounting for increase in length of the derived elements from pith outwards at any one height; further, that successively larger fusiform initials are laid down nearer the apex during the period of greatest vigour so that at this time the longitudinal elements in a ring are longest nearer the apex. After this periodof maximum vigour is past, new fusiform initials are smaller and hence the longitudinal elements of the xylem are smaller above a certain height. Vessel-elements and libriform fibres within one growth ring are longer in the late-formed wood than in the early-formed wood irrespective of whether the vascular cambium is stratified or not. The variation in the length of the vessel-elements is correlated with the diameter of the vessel, those of the spring wood shortening as a result of the considerable swelling during differentiation. On the other hand, the libriform fibres elongate during differentiation. Variation in vessel diameters, ray height and ray volume show much the same trend as that of the vessel-elements. Apart from certain minor details, wood of different ages and that from different parts of the tree, shows no great histological difference.

A study was made of assimilates and ionic tracer movementalong the stolon of Saxifraga sarmentosa. When ionic tracers or ¹⁴C-sucrose are applied to the transloadting stolon they undergo a longdistance transport which is strictly polarised, with a short-distance movement which is symmetrical with respect to the point of application. The distribution along the axis after a suitable length of time shows an accurate exponential fall-off pattern, except with naturally assimilated or applied ¹⁴C-sucrose where it is fairly level. In this case the linear velocity of movement was estimated as about 20 cm h^-1. Transport of materials is readily reversible by interchanging the roles of parent and daughter plants as source and sink. The transpiration stream can be similarly reversed. By these means it was confirmed that ¹⁴C-assimilates and ¹³⁷Cs move in the phloem, whereas ⁸⁹Sr moves in the xylem. The sieve tubes are thus not inherently polarised in this organ. Inhibitors such as nitrogen, cyanide and DNP applied overlengths of 20--30 cm exert a more or less complete and reversible inhibition of the movement of ¹⁴C-assimilates and ¹³⁷Cs. Cyanide gas is considerably more effective than solution. A variety of approaches, including the use of ¹⁴C-cyanide, confirmed that the inhibition was effective in the sieve tubes themselves, and not merely at the terminal sites. Electron microscopic examination showed that callosing in the sieve plates was not involved; however, in all cases, including the normal one, the sieve plate pores appeared blocked with P-protein. Tests with Valinonycin which possesses a particular affinity for potassium, were ineffective. This may have been due to its insolubility in water and to the large size of the molecule. Comparative studies were carried out by applying two ionic tracers together. These gave consistent and precise results in termsof the slopes of the plots log activity against distance Of the tracers used (²²Na, ⁴²K, ⁸⁶Rb, ¹³⁷Cs and ⁸²Br) the comparison of ¹³⁷Cs and ⁸²Br was perhaps the most interesting; the anion is transported more or less equally with the cation, but its lateral leakage appears to be less. The results of this study provide a well-established case for the following conclusions relevant to the controversy over mechanism: (1) Phloem transport in the stolon is strictly unidirectional. (2) Nitrogen, Cyanide and DNP exert a strong effect localised in the sieve tubes themselves (as well as at the terminals); it is reversible. (3) The inhibition of transport is not due to callose blockage. (4.) Anion transport follows a similar pattern to cation. These conclusions are very adverse to the Munch hypothesis, and to any diffusion-analogue theory; they favour a theory of activemass flow. The electro-osmotic theory faces the difficulty of anion transport.

A brief historical review and summary of recent work on leaf movement is presented. The leaf movements of Linum usitatissimum were studied by measurements of the angular position of the sub-opposite pair of leaves at the second node of the seedling. Oscillatory up and down movements of these leaves continue for about three days in normal light:dark cycles but cease as the leaves mature. In continuous bright light a rhythmic movement persists for two cycles. In prolonged darkness no further movement occurs after the first nyctinastic cycle. A rhythmic movement in darkness can be induced by short (4 h) periods of light and when the light is given in late subjective night the phase of the rhythmic movement is advanced; a delayed movement is induced by a short light period given in the early subjective night. These phase responses of the leaf movement rhythm are considered in relation to the mechanism of entrainment to a non-diumal (20 h) cycle of 10 h light and 10 h darkness. Evidence is presented for the involvement of the phytochrome system in the regulation of the leaf movement. Induction of rhythmic nyctinasty by short light periods in the early part of the subjective day can be reversed by short (15 min) treatment with far-red light, which is itself reversible by subsequent red irradiation. Red light breaks during darkness will induce rhythmic nyctinasty when given at appropriate times in the subjective day. At the close of the day the leaves are not sensitive to red light which is perceived, or darkness, but blue light delays the rising leaf movement until the rhythmic control overrides this effect of light. These results are discussed in relation to results recently published by other workers on movement of leaves possessing pulvini. Anatomical study of the basal part of the leaf of Linum confirms the absence of a pulvinus.

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