500 



NATURE 



[December 15, 1921 



blade oT a knife, the knife-edge pointing up-wind 

 and the parent leaf forming the back. 



On the largest of the ice-knives, which approached 

 an inch in width, a regular striped pattern was evi- 

 dent, even from a distance of two or three yards, 

 and closer inspection showed the same stripes on 

 every one. These stripes were three in number, pale 

 grey near the leaf, and again grey towards the edge 

 of the knife, with an intervening zone of clear or 

 yellowish ice. The grey stripes were evidently due 

 to the presence of smoke in the cloud-mist during 

 the daytime on November 25 and 26, while the clear 

 zone between had been formed from the cleaner night- 

 wind. 



At Edale Cross, 1750 ft. above sea-level, the un- 

 damaged knives pointed their edges due east magnetic 

 or directly to the centre of Sheffield. The inter- 

 vening distance is more than sixteen miles, of which 

 the first fourteen miles traverse some of the wildest 

 moorland in England. Though I became familiar 

 with the fouling eflfect of our industrial towns even 

 on this lonely hill-country during nearly two years' 

 residence there, yet this particular example seemed 

 strange enough to be recorded. 



W. Lawrence Balls. 



The Orchard House, Bollington Cross, 

 near Macclesfield, December i. 



An Oyster Spat (1921) with Mature Male Sexual 

 Products. 



In the course of experiments carried out this year 

 (192 1) on the rate of growth of the slipper-limpet in 

 the River Blackwater, Essex, I obtained a number of 

 oyster spat on shells put in a floating raft and on 

 clean scallop shells which were put in the water on 

 June 9. These clean scallop shells were strung out 

 on tarred rope at low water and kept suspended above 

 the ground by tying the rope to stout branches of 

 trees driven vertically into the ground. On Novem- 

 ber 16 the shells were taken out of the raft and from 

 the shore and the new summer growth, including 

 young oysters, examined and measured. Many of 

 the young oyster spat measured about an inch In 

 diameter (length anatomically) and an inch in depth 

 (height anatomically). Some of these living spat were 

 examined microscopically on the beds, and a few were 

 taken back to the Plymouth Laboratory for careful 

 examinatipn in the living state. 



In one of the spat examined at Plymouth a few ripe 

 sperm-morulae were found, which disintegrated into 

 active _ separate sperm while under observation under 

 the microscope. In order to confirm this interesting 

 observation microscopic sections were prepared, and 

 some ripe sperm-morulae found actually in the gonadial 

 tubes. Itis therefore established that oyster spat may 

 produce ripe sexual products in the year in which thev 

 are born. In the experiment described here the oyster 

 which gave mature male products had a maximum 

 age of twenty-three weeks ; it was probably younger 

 than this, but there is no means of determining how 

 much younger it actually was. In a paper bv the 

 present writer (" Sea-temperature, Breeding, and Dis- 

 tribution of Marine Animals," Journ. Marine Biol. 

 Assoc., vol. 12, No. 2, p. -^52, iQ2o) it was predicted 

 that "young specimens of the European oyster {O. 

 edulis) will be found to be sexually mature in the 

 summer in which they are spawned In those situations 

 where high temperatures obtain for a few months." 

 The exceptionally warm summer of 192 1 resulted In 

 an unusually long breeding period for oysters and in 

 "high temperatures" prevailing over English oyster- 

 beds for several months. In 1921, therefore, the 



NO. 2720, VOL. loK 



conditions over English oyster-beds compared with 

 normal conditions in such oyster-breeding places as 

 Arcachon, Taranto (Italy), and the spatting pools in 

 Norway. 



The observation here recorded is of considerable 

 academic interest, and might become of practical 

 interest when confirmed — as I have no doubt It can 

 be — In a large number of specimens. Between Sep- 

 tember and November this year I have been able to 

 examine only eight large 192 1 spat. Including those 

 mentioned above. Most of them gave indications of 

 developing sperm, but in such small quantity that 

 microscopic sections are required of them all for con- 

 firmation. The young oyster containing ripe male 

 sexual products was not ripe in the sense that all 

 the gonadial tubes at the surface of the body were 

 crammed with tailed sperm-morulae; and although 

 there can be no doubt that the sperm from this young 

 oyster were mature and were capable of effecting 

 fertilisation under suitable conditions. It is not thought 

 probable that ripe sperm are actually extruded by such 

 small forms. There can be no doubt, however, that 

 similar spat would be fully mature in the following 

 summer, and would certainly breed at least as males. 

 This experiment, which was made possible by a grant 

 from the Royal Sogiety, is being continued, and should 

 yield — together with a large amount of spat now 

 isolated from other localities — further useful material 

 for investigating the sex of oysters at first maturity. 

 The gonad of the young oysters examined In all cases 

 Indicated maleness, but the number examined is 

 far too few to be of any value statistically. It is 

 desirable to know the sex at first maturity of 1000 

 young dysfers, and as young oysters have been more 

 plentiful In most localities this year than for a good 

 manv years, It is probable that spat born this vear 

 can be Identified with certainty now and isolated for 

 examination next year. The sex of 192 1 spat could 

 be more easllv determined In a large number of In- 

 dividuals next summer, and If results were recorded 

 with accuracy from many different localities we should 

 obtain a total of Information which would help to 

 clear up the doubtful question of the sex of the 

 European oyster at its first maturity. 



J. H. Ortox. 



Marine Biological Laboratorv, The Hoe, 

 Plymouth, December 5. 



The Flight of Thistledown. 



My attention has been directed to the letters In 

 Nature of October 20 and November 10 on the flight 

 of thistledown. The explanation of the phenomenon 

 observed seems to me to lie in the very slow air- 

 currents which are sufficient to raise the pappose 

 structures of many Compositae, especially when it is^ 

 only the pappus which Is raised, the fruit having fallenf 

 off. 



Interested readers will find the hydrodynamics o^ 

 fruit dispersal by wind discussed In some detail 

 my " Origin and Development of the Compositae '^ 

 (Wheldon and Wesley, London, 19 19), and mor 

 briefly In my "Text-book of Botany" (Churchill 

 London, 192 1). Some of the results given may be 

 general Interest, e.g. the minimum winds which wi^ 

 raise the following pappose fruits (with fruit at 

 tached) are : dandelion, i m.p.h., groundsel, | m.p.hj 

 The pappus alone would be lifted by a much sIow( 

 wind, as Is shown by the fact that a current of air 

 moving at 059 m.p.h. is sufficient to blow the com- 

 plete fruit of coltsfoot for a considerable distance hori- 

 zontally. James Small. 



Department of Botany, Queen's University, 

 Belfast, December 2. 



