58 



NA TURE 



[July 8, 1922 



which alone is as large as many an atoll, is thus shown 

 not to be underlaid by an abraded platform, there 

 is no sufficient reason for thinking that the southern 

 compartment, or indeed any other barrier reef or atoll 

 lagoon in the whole Pacific, has any such smoothly 

 prepared foundation. Surely if the flatness of the 

 floor and its normal depth in both compartments of 

 the Tagula lagoon have been brought about in a 

 region of instability, and without the aid of abrasion 

 in furnishing a smooth sub-lagoon platform, there 

 is no sufficient reason for assuming that other flat 

 lagoon floors of ordinary depth can have been pre- 

 pared only on smooth platforms abraded at a standard 

 depth across still-standing islands. It is possible 

 that the Glacial lowering of the ocean surface by a 

 moderate amount may have contributed, in a manner 

 that I have suggested elsewhere, 6 to the production 

 of many lagoon floors 30 or 40 fathoms in depth ; 

 but Glacial changes of ocean level do not seem other- 

 wise to have left recognisable marks of their occur- 

 rence in the Louisiade archipelago. Crustal deforma- 

 tion has been dominant ; and the great changes of 

 shore lines thus determined appear to have been 

 merely played upon by the inferred oscillations of 

 ocean level during the Glacial period. 



This discussion is believed to show that, apart 

 from such changes of ocean level as are inherently 

 probable although they are not well-known either 

 in amount or in effects, the assumptions of the Glacial- 

 control theory are not applicable in the production 

 of Tagula reef and lagoon floor ; and hence we 

 may fairly conclude that these assumptions are not 

 essential to the production of similar reefs and 

 lagoon floors elsewhere. This argument, in which 

 the evidence furnished by one outspoken witness for 



6 " Problems ; 

 1916 (565). 



ated with the Study of Coral Reef," Sci. Monthly, 2, 



Darwin's theory and against the Glacial-control 

 theory is given wide application, would not be valid 

 if other witnesses were equally outspoken elsewhere 

 against Darwin's theory and for the Glacial-control 

 theory ; but such is not the case. It must be 

 remembered that the two main postulates of the 

 Glacial - control theory, namely, long - continued 

 stability of reef foundations in the mid-Pacific and 

 the abrasion of sub-lagoon platforms by the lowered 

 Glacial ocean, are not based on direct evidence but 

 are assumed because they are supposed to be neces- 

 sary for the explanation of smooth lagoon floors of 

 standard depths. Not a single example of an abraded 

 platform has been discovered under recently uplifted 

 reefs ; and a large number of mid-Pacific islands 

 which have a decipherable recent history are found 

 not to have been long stable but to have suffered 

 various changes of level. In other words, where 

 other outspoken witnesses are found, their testimony 

 is, like that of Tagula, for Darwin's theory of up- 

 growing reefs on subsiding foundations of whatever 

 shape. A large number of examples of this kind 

 could be adduced if space permitted. 



But although the inhibition of reef growth and the 

 resulting abrasion of low-level platforms by the 

 Glacial ocean thus appear to be excluded from the 

 greater part of the coral seas, it is highly probable 

 that reef-building organisms may have been weakened 

 or killed and that abrasion of platforms may have 

 taken place around islands near the margin of the 

 coral seas ; and at least some of those islands ought 

 now to show plunging cliffs in evidence of their 

 possession of submerged platforms ; but even there 

 the islands need not have been stable. This aspect 

 of the coral-reef problem is examined in an essay 

 submitted to the Geological Society of America for 

 publication in its Bulletin. 



Root Respiration. 1 



ALTHOUGH so much work has been done on the 

 question of root respiration, it is only within the 

 last few years that the importance of the air content 

 of the soil in this connexion has been clearly demon- 

 strated. With the growth of ecological work has 

 come the indication that this air content is a primary 

 factor in many habitats and a controlling one in wet 

 soil and water, but even yet this is not generally 

 recognised. 



Mr. F. E. Clements has endeavoured to clear the 

 ground for further research in this direction by 

 summarising the available information in all its aspects. 

 From the time of Mayou (1668) the necessity of oxygen 

 for root activity has been recognised, and numerous 

 investigations have since added to the bulk of evidence 

 with studies of germination, anaerobic respiration, and 

 the respiratorv behaviour of underground parts other 

 than roots. The excretion of carbon-dioxide by the 

 roots was first noted by Hales (1727), but the possible 

 excretion of other substances is still a matter of 

 controversy at the present day. Molisch first showed 

 that roots exhibit the phenomenon of aerotropism or 

 response to different concentrations of various gases. 

 This is of great significance in plants inhabiting bogs 

 and swampy land, as in order to obtain the oxygen 

 necessary for respiration they develop aerotropic roots 

 which run horizontally above the oxygen-free swamp 

 soil, as in Alnus, or rise vertically in the air, as in 

 A\ i' ennia. 



The composition of the soil air varies considerably 



with the nature of the soil, time of year, and seasonal 



• " Aeration ami Air Content : the Role of Oxygen in Root Activity," 



by Frederic E. Clements. Pp. 1S3. (Publication 315.) (Washington: 



Carnegie Institution, 1921.) 2 dollars. 



NO. 2749, VOL. I IO] 



changes, and is also affected by cultivation and plant 

 growth, which increase the carbon - dioxide and 

 diminish the oxygen in proportion. It has been 

 suggested by Bristol that the presence of algae may 

 also affect the soil gases. 



Anaerobic respiration is of much significance in 

 connexion with reduced oxygen supply. The general 

 effect of the reduction or absence of oxygen on respira- 

 tion is to reduce its intensity, but respiration under 

 anaerobic conditions differs with the species. Carbon- 

 dioxide and alcohol are the regular products of such 

 respiration, which is consequently regarded by most 

 investigators as essentially identical with alcoholic 

 fermentation when carbohydrates are present. Under 

 certain conditions acetic, formic, and lactic acids 

 arc excreted from the roots and other parts of flower- 

 ing plants. After considering the relation of photo- 

 synthesis, transpiration, and germination to oxygen 

 supply, Mr. Clements enunciates the general rule that 

 growth is decreased or prevented by the absence of 

 oxygen. The movement of protoplasm in plant cells 

 is stopped, and practically all tropistic responses are 

 suppressed. 



Field studies of aeration, approached from the 

 agricultural, pathological, and ecological standpoints, 

 corroborate the results of physiological investigation 

 as to the basic importance of oxygen for root activity 

 and the injury wrought by the accumulation of 

 carbon-dioxide. The practical importance of this 

 appears most strikingly in irrigated regions where 

 the common practice involves the use of too much 

 water, with consequent economic loss, due to the 

 production of an oxygen deficit in the soil. 



