Flora of Ashdown Forest - Botanical Society of the British Isles

129*Oxalis articulata. Pink-sorrel.•] -++-----'".t-Occasionally recorded scattered around the Forest, the common weedy pink sorrel in gardens,for instance as a garden escape near Brown's Brook (472.278), 1994, Flora meeting, andnaturalized in flower-beds at Oldlands Hall (475.275)' 1995, PD et al .Increasingly recorded in England (Rich & Woodruff 1996). Native in eastern SouthAmerica.4 5Oxalis acetosella. Wood-sorrel, Cuckoo's meat, Cuckoo-sorrel.Recorded in every tetrad on the Forest (Hall 1980).We have recorded it in all but 1ive squares, though it is not common. Plants with pinkflowers were noted at Newbridge, 1987, PW, It is widespread in Britain on many soil types,mainly in woodlands but also in the open where not droughted (Packham 1978). It iswidespread in the northern hemisphere.Wood sorrel is one of the best adapted woodland plants. It is evergreen and its ability togrow through the winter enables it to survive in quite deep shade. In this habitat the leavesare very thin, spreading the tissue to catch as much light as possible (Packham & Willis1977), Such leaves may over~heat when caught by sun~spots on the woodland floor, so ithas a clever mechanism to avoid damage; the leaves droop rapidly and move out of the way (this phenomenon was firstinvestigated by Bjorkman & Powles (1981) on a close relative O. oregana which grows in giant redwood forests in NorthAmerica). The movement is caused by specialised cells in the pulvinus (the junction between the leaflet and the stalk)which alter their size hydraulically. Experiments with O. acetosella by TR have demonstrated that when too much light isdetected in the pulvinus the leaves can droop within 6 minutes in bright light, and 15 minutes in duller light. If the light isfocused on the leaves but not the pulvinus it cannot be detected, and it is possible to give them enough light to bleach thechlorophyll without the leaves drooping. The response is initiated by blue but not by rep light. When the light is removedthe leaves take 30~45 minutes to move horizontal again. Young leaves respond faster than old leaves, and adjacentleaflets can act completely independently. The leaves also droop at night, a separate response called nyctinasty; Darwin(1882) demonstrated that it prevented radiation frost damage at night.Wood sorrel flowers freely in May, and the rest of the summer produces cleistogamous flowers which produce just asmany seeds. The capsules dehisce explosively ejecting the seeds a metre or more from the parent.GERANIACEAEGeranium.Many species are grown in gardens and become established for short period of time in the wild ~ one plant collected near agarden at Crow borough (506.322), 1994, PW was not identifiable, and there is one record of *G. macrorrhizum, 1994, ER(det. PW).*Geranium endressii. French crane's~bill.Tetrad 53A (Hall 1980).Two records from Forest Row as garden escapes by houses (one may even have beenplanted), one growing with the following species. The flowers get paler as the seasonprogresses.] --++-------'---1-- Locally naturalized in Britain. Native in the Pyrenees.4 5Geranium x oxonianum (G. endressii x versic%rj. Druce's crane's~bill.We have recorded this more fr~quently than G. endressii, and it seems to be more commonlygrown in gardens locally and is available in the garden centres. The plants are garden throwoutsor occasionally spread under their own steam (they are at least partly fertile).] --++-----------'-+--This hybrid has long been confused with its parents, and we hope we have recordedthem correctly because the commonest taxon nationally is supposed to be G. endressii. The4 5plants have pale pink flowers sometimes with deeper veins, and are often variable on thesame plant (Stace 1991). Plants sold locally as Geranium 'Claridge Druce' in garden centresare not pale pink or variable on the same plant.It originated in cultivation, and is also naturalized elsewhere in Britain.