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VEGETATION OF DRYING MUD AND RETARDED 

 GERMINATION. 



E. J. SALISBURY. 



The object of this communication is to call attention to the 

 vegetation of a type of habitat which, from its nature, is 

 apt to be of inconstant occurrence, and for which reason it 

 is the more desirable that observations should be placed on 

 record when opportunity offers. 



The past three years, and particularly 1919, were especially 

 favourable to observations on exposed mud owing to the low 

 level of the water in most ponds and reservoirs during the 

 summers of these years. 



The data here given are chiefly based on examination of 

 two reservoirs in Hertfordshire, viz.. Little Tring Reservoir 

 and Elstree Reservoir, and one, viz., the Welsh Harp in 

 Middlesex. All three supply the Grand Junction Canal, and, 

 owing to the withdrawal of water during the summer months 

 are liable to considerable fluctuations of water level. In 

 the year 1919 the entire portion of the Welsh Harp east of the 

 Midland Railway was drained completely, except for the 

 main channel, and the mud floor became covered with vege- 

 tation. In the same year the shallower part of Elstree Reser- 

 voir was similarly drained, though the plant covering was 

 not so continuous, whilst the deeper and larger area of water 

 was so much reduced that Potamogetons, such as P. perfoliatus 

 and P. coloratiis, were completely exposed. The first note- 

 worthy feature is the character of the succession, which was 

 essentially similar in all three localities. It should be stated 

 that the mud thus colonised appears to differ but little, the 

 observed natural water content being from 65 — 80%, with a 

 rich supply of electrolytes, including nitrates, and a reaction 

 which was slightly or even appreciably alkaline (Ph. 7-2 — 7*6). 



The pioneer on the very soft freshly exposed mud was 

 Vaucheria, the species, when found in fruit, being V. sessilis. 

 Near the waters edge, if no more water be withdrawn, the 

 Vaucheria may persist, but elsewhere it diminishes with the 

 decreasing water content of the surface crust. Occasionally 

 Cyanophyceae have been noted as pioneers accompanying or 

 even replacing Vaucheria. These appear to be mostly species 

 of Lynghya. 



The next phase in the succession is the appearance of 

 Botrydium granulatum. This alga occurs in such vast numbers 

 that, both at Little Tring and at the Welsh Harp, it was seen 

 covering many square yards like a carpet of green dew-drops, 

 but so closely packed that each individual had assumed a 

 hexagonal form in plan. 



In this connexion it is pertinent to recall the observation 



a921 Oct. 1 



