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ALONG THE southern and western coasts of the Florida peninsula occurs one of the greatest developments of mangrove swamp in the world. Rhizophora mangle L., the red mangrove, perching on its arched stilt roots, is the dominant species and the most marine. Another important component is Avicennia nitida Jacq., the black mangrove, which is restricted to the tidal zone and reaches its most luxuriant development on intertidal mud flats in more sheltered inland localities. Both species have a considerable tolerance for fresh water (Davis, 1943). Familiar to anyone who has visited the mangrove region are the air roots which protrude in great numbers from the mud under and around Avicennia bushes or trees. A single tree may produce several thousand of these air roots, usually 20-30 cm. high and a centimeter thick, soft and spongy, and studded with little whitish lenticels. In the mud they connect to radially-running main roots, which are also soft and spongy and contain large amounts of air (fig. 1). The vertical air-roots are often referred to as pneumatophores because it is believed that they may have the function of aerating the root system which lies buried in the mud. If one of the arching stilt roots of Rhizophora is dug out of the mud it is found to terminate in a bunch of long, spongy, finger-thick, and air-filled roots (fig. 2). The stilt root has a conspicuous development of lenticels, and one may wonder whether these might serve as ventilating ports for the mud roots. If they do, one might ask by what process the ventilation takes place, as it seems almost incredible that the voluminous roots down in the mud could be effectively ventilated by simple diffusion alone, through little lenticels high up on the stilt roots. One might also wonder how such a system, which gets its oxygen from the air, avoids getting jammed with nitrogen. However, before worrying over how the ventilation comes about one must first establish that such ventilation does take place, and this is the object of the present investigation. GAS EXCHANGE IN SUBMERGED ROOTS OF VARIOUS PLANTS.-Several investigators have analyzed the gases in roots of plants growing in water or mud which is poor in oxygen. In general these roots have been found to be aerated; how is usually not clear. Conway (1937), in a series of experiments on the sedge Cladium mariscus R. Br., found that air spaces were continuous from the leaves down into the roots. The gas connections were particularly free

A sterile germination study with seeds of some common phanerogamic water plants showed almost 100 per cent germination for seeds of Alisma plantago–aquatica, Baldellia ranunculoides and Nymphaea alba. Seeds of Potamogeton lucens could be germinated to about 40 per cent, seeds of Polygonum amphibium germinated sporadically while those of Cladium mariscus could not be germinated at all. Freshly harvested seeds of Alisma and Baldellia showed an ability to germinate at both 20°C and 35°C. A stratification period of one month at +4°C gave germination of all species tested, with the exception of Cladium. Potamogeton germinated in light only, the other species both in light and darkness. Treatment times for surface sterilization in disinfectants are given.

IT has generally been assumed that the abundant air spaces in the tissues of aquatic plants make it possible for the submerged parts to obtain the oxygen necessary for respiration even though the surrounding medium does not yield a sufficient quantity. The airspace system is assumed to be continuous throughout the plant and of such a nature as to allow gases to diffuse according to the concentration gradients at a rate sufficient to supply the oxygen demands of all the tissues. These assumptions do not seem to have been put to a direct experimental test, although several pieces of work bear on the problem from one side or another. There are researches such as those of Bergmann on the effects of oxygen lack on the roots of different types of plant and the relation of these effects to ecological distribution. Bergmann (I920) worked with eight or nine species, some of them land plants, for instance Pelargonium, Impatiens balsamina and Zea; some of them hydrophytes, for instance Ranunculus abortivus, R. sceleratus and Cyperus alternifolius. He found that with the land plants, submergence of the soil in which they were growing caused a decrease in water absorption by the roots and finally killed them. These effects could be prevented by aerating the water. The hydrophytes, however

Je reicher fossile Funde sich darbieten, je vollkommener sie konserviert wurden und je sauberer sie getrennt sind, desto feinere pflanzensoziologische Einheiten kann man aus ihnen feststellen. Hohere syntaxonomische Rangstufen, wie Klassen, Ordnungen oder Verbande, lassen sich leichter erkennen als Assoziationen. Dabei bleibt als Voraussetzung aber die Annahme einer gewissen Konstanz der Arten-Verbindung uber langere Zeitraume, die dann wohl in der Tat gegeben ist, wenn keine oder nur wenige andere Arten als heute vorkommen.