480 



NA TURE 



[October 7, 1922 



conditions will have a certain similarity to those 

 existing in the atmosphere. The difference in the 

 character of the circulation in the two cases could 

 scarcely fail to give some useful hints. 



Another illustration of the kind of flow to be 

 expected near the borders of the Trades may be 

 observed (although the analogy is not so close as in 

 the experiment) whenever a current of water flows 

 into a pond. The central part of the stream continues 

 on its course for some distance unbroken, but the 

 margins are bordered by eddies, which (looking down 

 stream) are right-handed on the right, and left-handed 

 on the left side, and consist of equal volumes of water 

 from the stream and from the pond wrapped together 

 after the fashion of a " roly-poly " pudding. When 

 once formed, they have a certain life of their own, and 

 follow erratic courses, often generating secondary 

 eddies further from the main stream. In general 

 their life is short, but occasionally vertical components 

 in the flow of the main stream give rise to components 

 in the eddies parallel to their axes, and in such cases 

 the vortices may be sustained and intensified. 



Much the same sort of action must be going on at 

 the borders of atmospheric currents, and it must 

 happen, especially in the turbulent regions, that 

 either on account of the general circulation or from 

 local causes, warm air will sometimes underlie colder 

 strata, and this is what is required to prolong the life 

 of eddies or vortices with vertical axes. 



It may be said with some confidence that tornadoes, 

 sand pillars, and waterspouts are due to local causes 

 of this kind, and it seems highly probable that the 

 deep barometric depressions which accompany the 

 greater storms have a similar origin depending on 

 inversions of level of the general circulation. In 

 referring to warm and cold strata, the temperature 

 must be supposed to be compared at the same altitude 

 since, so far as thermometric readings are concerned, 

 the upper air is always colder than that near the 

 ground. A. Mallock. 



9 Baring Crescent, Exeter, 

 August 10. 



The Conditions of Sex-change in the Oyster 

 (Ostrea edulis). 



In the issue of Nature for August 12, p. 212, 

 and in several previous numbers, Dr. Orton has given 

 some interesting information concerning the old 

 question of the breeding habits of oysters, especially 

 sex-change and its conditions. This problem has 

 been discussed in a certain number of ancient treatises 

 (Davaine, Van Beneden, Lacaze-Duthiers, Hoek, 

 etc.), but has been but little investigated in the 

 course of the last few years. During my work at the 

 Danish Biological Station I have, since 1919, been 

 making experiments and investigations on the biology 

 of the oyster in the Limfjord. As my results in 

 several respects confirm and amplify those of Dr. 

 Orton, I will give here a short account of some of 

 the most important. In the course of the winter a 

 more detailed paper will probably be published in 

 the Report of the Danish Biological Station. 



Dr. Orton confirms the observation, made by 

 Mobius, that in European oysters a specimen directly 

 after breeding produces spermatozoa, and I fully 

 agree with him. In several cases I have proved, 

 through experiments with oysters, in the shells of 

 which a little hole had been bored, that an oyster in 

 the course of less than a week changes from a female 

 to a male. 



Dr. Orton further mentions the interesting fact 

 that he has been able to state that an oyster born in 

 1 • .2 1 was spawning already in 1922 ; this phenome- 



NO. 2762, VOL. I IO] 



non he ascribes, and very rightly, to the high tempera- 

 ture of the summer 1921. I have investigated several 

 thousand oysters in the Limfjord : the youngest 

 female found by me was at least three years old, 

 which is no doubt due to the lower temperature of 

 the Limfjord. Neither did I ever find that oysters 

 had ripe spermatozoa in the summer in which they 

 were born ; in the Limfjord that phenomenon only 

 occurs in the following summer. Formerly the 

 earliest time for an oyster to breed was much dis- 

 cussed. If we examine from where the different 

 authors have obtained their material, it appears that 

 those who advocated early breeding had got theirs 

 from Southern France, while those who advocated 

 two to three years as the age for breeding had had 

 material from the English Channel and the North 

 Sea. 



From my experiments, and from the study of 

 previous papers on this subject, I have come to the 

 conclusion that the duration of the male stage depends 

 on temperature, so that the colder it is the longer the 

 stage lasts. At the temperature which ordinarily pre- 

 vails in the Limfjord (i5°-i6° in July), this stage will 

 last three to four years. The oyster, therefore, 

 breeds for the first time (the first stage being the male 

 stage) when it is three to four years old ; further, 

 every single oyster individual in ordinary circum- 

 stances of temperature breeds only every third or 

 fourth year, in especially cold years still less often, 

 in warm years more often. These phenomena, 

 together with the shorter duration of the female 

 stage, explain the fact that in a certain number of 

 oysters in the Limfjord we always find only a rela- 

 tively small percentage of females. This likewise 

 explains why the oyster breeds more sparingly the 

 further north it is, and decreases regularly in number 

 without any sharp boundary-line. 



The breeding of the oyster is in at least three 

 respects influenced by temperature. A high tempera- 

 ture increases the number of times an oyster may 

 breed in its life, it shortens the time which the breed 

 passes in the mantlecave of the mother animal, and, 

 according to Hagmeier, it shortens the pelagic larva 

 stage. R. Sparck. 



Copenhagen, September 5, 1922. 



Rise in Temperature of Living Plant Tissue 

 when infected by Parasitic Fungus. 



While engaged on some work connected with the 

 export of citrus fruits from South Africa to England, 

 we have come across a point of interest to plant 

 pathologists and bacteriologists which would seem 

 worth recording at this stage. 



In investigating the effects of inoculating oranges 

 and grapefruit with Penicillium digitatum we found 

 that a very definite rise of temperature took place 

 in the infected tissue. We are not aware of such 

 an observation having been made before in connexion 

 with the invasion of plant tissue by a parasitic fungus, 

 and it will be interesting to ascertain whether a 

 similar rise of temperature takes place in all cases 

 where living plant tissue is attacked by parasitic fungi 

 or bacteria. 



To what extent direct reaction of the host is 

 responsible for the rise of temperature is still to be 

 determined ; certainly no rise of temperature Was 

 observed when the host tissue was killed prior to 

 inoculation. Mercury-in-glass thermometers were 

 used in making these observations, but the employ- 

 ment of thermo-electric apparatus will naturally be 

 necessary to carry the investigations further. 



This observation of ours would seem to open up 



