SCIENCE-GOSSIP. 



199 



PLANTS OF THE BLACK POND. 



By H. P.. Gurry, M.B. 



^ T 7 E have done so much to deprive the ponds 

 and lakes around London of their natural 

 character, that a visitor to the Black Pond at 

 Oxshott, in Surrey, gazes with agreeable surprise 

 on a lakelet that still owes some of its most striking 

 features to nature. Brakes of the common reed 

 (Aru ndo phragmites) conceal its waters from the 

 view until one stands at its borders. The cotton- 

 sedge [Eriophorum polystachion) flourishes at its 

 margins, and the boggy ground is in places literally 

 carpeted with the sundew (Drosera). In its shallow 

 waters occur dense beds of Hypericum elodes, and 

 frequent patches of Potamogeton oblongus. Floating 

 masses of Scirpus fluitans rest on its surface, and in 

 the bog moss that fills its recesses and forms its 

 edges, there grow Hydrocotyle vulgaris, Viola palustris 

 and Calla palustris. Another interesting plant, 

 Pilularia globuli/era, lives in the pond, though I 

 only came upon portions of its fronds in the 

 floating drift. Its existence in the immediate 

 neighbourhood is recorded in Salmon's Flora of 

 Surrey. Those familiar with the Black Pond will 

 observe numerous omissions in my list, but enough 

 has been said here to illustrate this paper. 



The plant of strangest aspect here, is certainly 

 Calla palustris. At home in the swamps of Europe, 

 Siberia and North America, this aroid of the north 

 thrives in the bog-moss at one corner of the pond, 

 and there flowers and matures its seed seemingly as 

 it does in Lapland. According to Watson's 

 "Topographical Botany" (1883), this plant was 

 supposed to have been introduced here by a medical 

 man, whose labours in this direction were not 

 appreciated by that eminent botanist. However, 

 in the spring of 1894, I committed a similar 

 impropriety by throwing numbers of the seedlings 

 in the large pond in the Home Park that lies near 

 the Kingston Gate. This plant has a larger 

 geographical range and considerably greater 

 opportunities of dispersal than are possessed by 

 A ruin maculatum. The berries in the last case can 

 only float for a week or two, whilst the seeds 

 sink ; nor do I think that birds would often assist 

 in distributing the plant by swallowing the seeds. 

 On the other hand the seeds of Calla palustris float, 

 as I have found, for an indefinite period both in 

 fresh and sea water, and retain their powers of 

 germination after floating for six months in the 

 sea. But most probably in aquatic birds we have 

 its principal means of dispersal. The seeds would, 

 no doubt, be able to pass uninjured through the 

 digestive system of a bird, especially if it had 

 previously satisfied its hunger to repletion. The 

 mucilage that invests the seed freshly fallen from 



the plant enables it, as I have ascertained by 

 experiment, to adhere firmly, on drying, to a bird's 

 plumage. The probability of this mode of trans- 

 portal by adhering to feathers, has been also 

 pointed out by M. Kolpin Ravn. This gentleman, 

 in his paper on the floating capacity of the seeds 

 of aquatic and marsh plants (Saertryk af Botanisk 

 Tidsskrift, 19, Bind, 2 Hefte, Copenhagen, 1S94), 

 supplies an explanation of the great buoyancy of 

 the seeds of this plant (a character which he also 

 observed) in the account he gives of the structure 

 of their integuments, in which occur very large air- 

 cells containing crystals of oxalate of lime. 



Another plant of this locality, Viola palustris, 

 presents an interesting theme to the student of 

 geographical distribution. Of our six species of 

 violets, it is only this species that finds its habitual 

 station in a marsh that occurs in North America. 

 Like the other species, Viola palustris produces 

 seeds that have no buoyancy, and we must connect 

 its station with its wide distribution, and its wide 

 distribution with aquatic birds. It resembles Viola 

 canina in ejecting its seeds with considerable force, 

 the process being the same in both cases. Some- 

 times during the two or three days preceding the 

 dehiscence of the fruit, the peduncles display 

 marked movements of circumnutation, which cease 

 a few hours after the discharge of the seeds. Since 

 the plants often lie partly concealed in the bog- 

 moss, these movements may aid the free propulsion 

 of the seeds into the air. Sir John Lubbock, in his 

 "Flowers, Fruits, and Leaves," gives some interest- 

 ing particulars of the various methods employed by 

 our violets in dropping or in ejecting their seeds. 



It is not easy to see what real advantage, in the 

 matter of distribution, certain plants acquire from 

 the faculty of ejecting their seeds. Not the least 

 important section of Kerner's " Pflanzenleben " is 

 that which is devoted to this subject ; and the main 

 point to grasp in perusing those pages is the total 

 dissimilarity of the processes employed, as for 

 instance in our species of Viola and Oxalis. I have 

 spent many hours in watching the operation so 

 prettily performed by Montid fontana. Here the 

 propelling power is to be found in the instantaneous 

 springing up of the valves of the capsule. The 

 three valves lie flat back, leaving the three seeds 

 exposed like eggs in a nest. A rapid movement 

 follows, which the eye cannot detect, and one sees 

 the fruit empty and the valves standing erect, like 

 three scrolls, in the centre. Under ordinary 

 •conditions about one-third of the ejected seeds fall 

 back amongst the little plants, and only ten per 

 cent, fall more than a foot from the edge of the tuft, 



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