Summary Vesicular‐arbuscular (VA) mycorrhizal infection of red clover grown in phosphate deficient soils enhanced the concentration of P in the tissues, stimulated growth of root and shoot but reduced the root/shoot ratio. Addition of phosphate to well below the optimum level also stimulated growth and enhanced P status of non‐mycorrhizal plants, but their yields and P concentrations were much smaller than those of mycorrhizal plants and their root/shoot ratios were unaffected. The hydraulic conductivities of the root systems were very much higher in mycorrhizal than in uninfected plants. This could be partly attributed to the greater lengths and diameters and hence total surface areas of the mycorrhizal roots. Per unit length of root, the conductivities of the mycorrhizal roots were still two to three times higher, suggesting that this was mainly due to hyphal growth in the soil. When soil water was not limiting, the lower root resistances combined with larger leaf surface areas and possibly also lower leaf diffusion resistances resulted in very much higher transpiration rates and fluxes in mycorrhizal than in non‐mycorrhizal plants. Under conditions of water stress, however, the leaf diffusion resistances of mycorrhizal plants were higher and transpiration fluxes lower compared with uninfected plants but, because of their higher total water demands, they wilted more rapidly. Mycorrhizal plants were able to extract soil moisture down to lower water potentials than non‐mycorrhizal plants (difference about 1 MPa) but, possibly because of their lower leaf water potentials and higher root conductivities, they recovered turgor more rapidly than non‐mycorrhizal plants when soil water was restored. Thus, the mycorrhizal habit is an advantage to the host plant in times of moisture stress.

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SUMMARYThe fine structure of the cuticle of mature needles of Pinus radiata has been studied from electron micrographs of carbon replicas of the needle surface. The whole of the needle surface appears to be covered by a layer of wax from which, with the exception of the inner surface normally enclosed by the sheath, arise numerous tubular waxy outgrowths similar to those previously observed in P. sylvestris. Unlike P. sylvestris, these outgrowths cover only a portion of the exposed needle surface and the underlying wax layer ranges in appearance from smooth to slightly granular, with no indication of a plate-like structure.When the needles were immersed in water, very little was taken up into the basal region of the needles normally enclosed by the sheath compared with the exposed surfaces; the tips of the needles showed the highest permeability to water. The inner surfaces of the needles were consistently more wettable than the outer surfaces, and the surfaces normally enclosed by the basal sheath, corresponding to the youngest regions of the needle, were much less wettable than the exposed surfaces. The inner enclosed surface was the least wettable of all; this may be due to differences in the chemical composition of the wax, besides the absence of weathering.When young needles were prematurely exposed by removing the basal sheath, tubular, waxy outgrowths appeared, similar to those found on the exposed surfaces of the mature needles; when young needles were bound together to prevent them from parting, an entirely different structure developed consisting of minute granules forming a reticulum over the surface.The significance of the above findings is discussed in relation to the reputed relatively high cuticular transpiration of P. radiata and its ability to absorb water directly into the foliage from precipitation; it appears that this species is well adapted to the moist atmospheric conditions characteristic of its native habitat in the Californian 'fog belt' region.

SUMMARYMarked variations in secondary vascular growth of detached Fraximis e.xcelsior L. stems were obtained with changes in water potential and in IAA and GA concentrations in the culture medium. The width of new tissue, the number of fibres and the mean fibre diameter increased with IAA concentration up to iomg.P' and generally with GA concentration up to ioomg.P', but by far tbe greatest influence was tbat of water potential wbich, especially between zero and -O.I J.g~ , significantly depressed cell division and expansion. Expansion of cambial initials to a diameter of about 6 n appeared to be necessary before active cell division commenced.essel development was particularly sensitive to IAA supply and tbe amount of IAA required for the formation of groups of vessels increased as tbe number of constituent vessels increased and as the water potential was reduced. Gibberellin bad no significant effect on tbe number of vessel groups formed, but appeared to increase tbe number of vessels formed in any group. Vessel formation may occur without cell division in an initially dormant cambium. Increasing the IAA supply led to increasing vessel cross sectional areas, but progressively less effectively as the water potential was reduced; tbe marked reduction in vessel lumen area between zero and -O.I J.g~' potential suggests tbat vessels formed at reduced water potentials may bave different metabolic patterns tban tbose formed at zero potential from an active cambium.The significance of tbe depressing effect on cell division and expansion of even a slight reduction in water potential is discussed in relation to wood development in trees growing under conditions of moisture stress.

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