Vegetation of Drying Mud and Retarded Germination. 331 



A similar history attaches to Limosella aquatica. First 

 recorded in Hertfordshire by Coleman in 1846 at Elstree 

 Reservoir, it was again found in a drying pond in 1856, and a 

 period of 61 years elasped before it was once more met with 

 by the writer at Little Tring. Here it was unknown previously, 

 but yet occurred in such quantity as to give a lawn -like 

 appearance to the mud which it covered. Only once has it 

 been seen by the writer growing submerged, and that was 

 due to a sudden rise in the water-level after the plants had 

 become established. 



The third species of importance is Runiex limosns. Like 

 the preceeding, a rare British plant, yet in 1919 it was the 

 dominant plant on a zone some 6-8 feet wide, encircling the 

 whole periphery of the Eastern position of the Welsh Harp, 

 besides being scattered over the central area amongst other 

 species. In this same year Mr. Carleton Rea recorded a 

 similarly sudden appearance in quantity of the very rare 

 Elatine hydropiper, on exposed mud near Droitwich (c.f., 

 Journ. Bat., p. 323, 1919). 



Other data might be quoted, but these suffice to emphasise 

 that we are here dealing with rare species which suddenly 

 appear in great profusion under conditions that are attained 

 during periods of drought. One knows from actual observa- 

 tion that a certain zone, if bare mud is exposed every year 

 without, however, producing any effect proportional to the 

 diminished area as compared with that during exceptionally 

 dry seasons. This is probably due to the comparatively 

 short period of exposure when the season is not very dry, and 

 the liability of the narrow zone to periodic inundation. That 

 is to say, the evidence points to the necessity for prolonged 

 and continuous exposure. 



Other common species exhibit the same phenomenon, 

 though in their case the effect is not so striking. A good 

 example is furnished by Chenopodium rubrum v. blitoides, 

 which formed over 70% of the dense plant covering on the 

 mud floor of the Welsh Harp in 1919. 



As to the probable explanation. Gupply showed that 

 the germination of some aquatic seeds may be delayed four 

 or five years without impairing their vitality (c.f. H. B. 

 Gupply, On the postponement of the germination of the 

 seeds of aquatic plants, Proc. Roy. Phys. Soc, Edinburgh, 

 Vol. XHL, pp. 344-359, 1894-97). Whether delay is due to 

 mechanical pressure exerted by the seed coat, as suggested by 

 Crocker (W. Crocker, 'Germination of Seeds of Water Plants,' 

 Bot. Gaz., Vol. XLIV., pp. 375-380, 1907, and W. Crocker 

 and W. E. Davis, Delayed Germination in the seed of Alisma 

 Plantago, Bot. Gaz., Vol. LVHL 285-321, 1914), or to lack of 

 chemical stimulation, as suggested by Fischer (A. Fischer, 



1921 Oct. 1 



