. The Canadian field-naturalist. 1976 Notes 185. FiGL'RE 1. Selection of abscissed twigs of Acer saccharinuni, collected from trees adjoining pavement. Ottawa, Ontario, 3! May 1975 (Savile 5141); photographed by Ken W. Spicer. Inset: detail of twig base; photographed by Modra Kaufert. abnormally wet; and no twig abscission was noticed in any of these trees. In 1975 the weather was excep- tionally warm and dry from late April to late May. Again no significant abscission occurred in the trees behind the house; but the trees in front of the house shed many twigs up to at least 35 cm long, mainly
. The Canadian field-naturalist. 1976 Notes 185. FiGL'RE 1. Selection of abscissed twigs of Acer saccharinuni, collected from trees adjoining pavement. Ottawa, Ontario, 3! May 1975 (Savile 5141); photographed by Ken W. Spicer. Inset: detail of twig base; photographed by Modra Kaufert. abnormally wet; and no twig abscission was noticed in any of these trees. In 1975 the weather was excep- tionally warm and dry from late April to late May. Again no significant abscission occurred in the trees behind the house; but the trees in front of the house shed many twigs up to at least 35 cm long, mainly in the last half of May. Figure 1 shows that the twigs oi Acer sacchariiium are cut off by a typical conical corky abscission layer, such as Millington and Chaney (loc. cit.) illustrate for Populus and Quercus. Abscissed twigs oi A. ruhrum are generally somewhat smaller but otherwise iden- tical in form. Water supply to the twigs seems generally to be cut off before the buds burst; but, as the lowest twig in Figure 1 shows, the vessels may remain intact until just before the twig falls, allowing partial expansion of the leaves. Acer ruhrum ranges in habitat from swamps and lake shores to arid south-facing rocky hillsides. Twig abscission in this species seems to be an adaptive device, whereby shedding of a proportion of twigs saves the tree from developing an interveinal scorch of all its leaves, such as is often seen in sugar maple {A. saccharum) under conditions of stress. Acer sac- charinunu which is closely related to A. ruhrum, grows most commonly on moist bottom land exposed to prolonged spring flooding. E. Jorgensen suggests that this flooding causes a physiological drought and twig abscission. The abscission habit is perhaps general in section Ruhra Pax. and may indicate an ancestral species that evolved under conditions of frequent water stress. D. B. O. Savile Biosystematics Research Institute, Central Experimental Farm, Ottawa, Ontario K1A0C6 Received 2 October 1975
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