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Ectotrophic mycorrhizas have been studied intensively and their value to some tree species is well known. Endotrophic mycorrhizas have received less attention and their potential value to plants has only been fully substantiated within the last 12 years. Responses of the host to mycorrhizal infection seem always to be associated with improved phosphorus nutrition of the plant. The mechanism whereby this occurs is considered. There is evidence that the ultimate limitation on phosphorus uptake by a simple cylindrical root is the diffusion impedance in the soil around it, and that widely spreading hyphae effectively short‐circuit this impedance. Some data on hyphal length, and estimated net flux of phosphorus through the hyphae are given, with a discussion of possible mechanisms driving this flux.

SUMMARYFour vesicular-arbuscular endophytes were inoculated into onions cultured in soil in a growth chamber. Dry weight, root length, infected root length, phosphorus content and quantity of external mycelium were measured at intervals. Three endophytes produced similar hyphal inflows, growth increments and external mycelium. One endophyte produced no growth increase in the host, and had little external mycelium and slow increase in percentage infection.

SUMMARY Tracings of electron micrographs of mycorrhizal roots of onion were measured using an image‐analysing computer and the results were used to derive estimates of the volume occupied by the fungal arbuscules, the increase in volume of host cytoplasm after infection and the area of interface between fungus and host. The average lifespan of an arbuscule was deduced from the proportions of living and senescent arbuscules measured in light microscope preparations. These measurements were used to evaluate two hypothetical mechanisms for transfer of phosphorus from fungus to host. It was concluded that digestion of the fungus by the host was not likely to be a significant factor in phosphorus transfer. Transfer of phosphorus across the membranes of living fungus and host appeared to be a more probable mechanism.

This is a completely revised edition of the previously titled Solute Movement in the Soil-Root System. It describes in detail how plant nutrients and other solutes move in the soil in response to plant uptake, and it provides a basis for understanding processes in the root zone so that they can be modeled realistically in order to predict the effects of variations in natural conditions or our own practices.

SUMMARYLeek plants (Allium porrum) were grown on partially sterilized soil either inoculated (M) or not (NM) with the vesicular-arbuscular mycorrhizal fungus, Glomus mosseae. They were pulse-fed with ^"COj in an apparatus which allowed CO^ subsequently respired either by the shoots or by the roots plus soil to be separately monitored. There were three experiments. In two, plants were harvested 48 h after labelling and in the third after 214 h. At harvest, the distribution of^C between shoot,, root, soil organic matter and root washings was measured. Similar growth curves for M and NM plants were obtained by supplying extra phosphorus to the latter, so that C distributions for both treatments could be compared directly. In all three experiments, about 7 % more of the total fixed C was translocated from shoot to root in M plants compared to NM plants. In the third experiment, this extra translocate could be accounted for by increased root respiration plus increased loss of C to the soil but, despite this drain, M and NM plants had equal rates of C assimilation per unit of leaf area. However, shoots of M plants had a lower content of dry matter and hence higher assimilation rates expressed on a dry matter basis.Increased hydration is suggested as a mechanism whereby leaf area and hence C assimilation increases in mycorrhizal plants and which offsets the effects of the drain imposed by the mycorrhizas.

S U M M A R yplants (Allitmi porruttt L.) were grown in a controlled environn-ient on a mixture (2: 1 w/w) of-y-irradiated (1-0 Mrad) sandy loam and sand, at six concentrations of bicarbonate-soluble pbospborus (P) ranging from 22 to -'44 mg P kg ' (soil basis). Inoculum of tbe \-esicular-arbuscular (V.A) mycorrbizal fungus Glottttis tnosseae (Nicolson & Gerdemann) (jcrden-iann and Trappe was placed (M) or not (NM) in a layer 3 cm below tbe soil surface.At intervals of 10 d, lengtbs of main axes of roots and tbeir lateral branches, and of tbe segments of infection i them, were measured. From these data we calculated tbe mean (barvest interval metbod) rates of linear ion of root tips and of infection fronts for eacb member of root, averaged over tbe wbole root systen-i. Tbe mean delay, d, between a root encountering tbe layer of inoculum and tbe subsec^uent formation of internal infection, was also derived.Addition of P to soil did not affect rates of extension ot roots, but increased tbe initiation of main axes and laterals. Infection segments extended twice as fast in laterals as in main axes. At low rates of addition, P did not affect fungal behaviour but increased the length of root available for colonization.Wben bicarbonate-soluble P exceeded 140 mg kg ', tbe rates of extension of infection fronts in botb n-iain axes and laterals were approximately balved, and d was considerably increased. Tbe density (tbe area ratio of fungal •^o bost tissue in a longitudinal squasb) of tbe byphae and arbuscules respectively, and tbe number of entry points P*^' -unit lengtb of root, were greatly reduced by added P. However, tbe ratios ot nun-ibers of entry points/bypbal density and of arbuscule density/hyplial density were unaltered, and tbe morpbology of tbe fungus was not noticeably affected.It is probable that fortnation of entry points was tbe rate-limiting step for colonization, and that tbis rate was ••educed by added P.