■ 



February 2^, 1922] 



NATURE 



237 



demonstrated by one of us (H. H, Dixon, Notes from 



I^HT the Botanical School of Trinity College, Dublin, vol. 2, 

 ^^n}p. 5 et scq.) in the case of Tilia microphylla, Syringa 

 ^^Pvulgaris, Salix viminalis, and Philadelphus sp. 

 Similarly, Luise Birch-Hirschfeld {Jahrb. /. wiss. Bot., 

 Bd. 59, pp. 171 et seq.) has shown that a solution of 

 lithium nitrate may be carried considerable distances 

 downwards in the reversed transpiration stream of 

 various trees, shrubs, and herbs. 



Naturally the question obtrudes itself as to how a 

 downward current of dissolved carbohydrates is pro- 

 duced in the wood which is also the normal channel 

 of the upward transpiration current. The subject 

 calls urgently for investigation, and it may not be out 

 of place to mention some of the hypotheses which 

 must be tested. 



If it could be shown that the wood of the vascular 

 bundles were divided longitudinally by more or less 

 impermeable partitions into isolated tracheal strands, 

 we might suppose that the tension developed in the 

 water in some of these strands by transpiring cells, 

 while raising the sap in these, might draw down solu- 

 tions ejected by adjacent cells in neighbouring strands. 

 No such vertical partitions have been described, unless 

 the late summer wood or the vertical plates of 

 parenchyma in the leaf veins and petioles be regarded 

 as such. With such longitudinal partitions a con- 

 tinuous and contemporaneous upward and downward 

 transport might be developed. 



An intermittent downward flow might be explained 

 if we could obtain evidence of a periodic or occasional 

 development of permeability in the protoplasmic 

 utricle of the transpiring cells. This might be de- 

 veloped in response to any stimulus, e.g. the mounting 

 of tension in the adjacent sap above a certain limit. 



The periodic mounting of tension with consequent 

 contraction of the stem, which is indicated in the 

 experiment on the potato plant quoted above, has been 

 established by the elegant observations of Mallock 

 (Proc. Roy. "Soc., 1919, vol. 90, B, pp. 186-91) and 

 of MacDougal ("Growth in Trees," Carnegie Institu- 

 tion of Washington, Washington, 1921). These latter 

 show graphically the change of volume of the woody 

 stem of forest trees corresponding to the diurnal 

 period. 



Again, the deposit of dew on the transpiring cells, 

 in conjunction with a high tension, might determine 

 a downward flow in the tracheae, and, with suitable 

 modification of the permeability of these cells, this 

 downward stream might be charged with dissolved 

 carbohydrates. 



It is quite evident that the tension assumed here 

 may be developed by temperature changes of the wattr 

 in the woody tissues and by recovery from flexure 

 just as well as by evaporation. That tension is reallv 

 responsible is indicated by the experiments quoted 

 above, and also by the fact that no transport occurs 

 from cut leaves floating in water. 



If the view that the longitudinal transport of organic 

 substances takes place in the tracheae is established, 

 speculation naturally arises as to the function of the 

 bast. While the form and arrangement of this tissue 

 seein to preclude any important longitudinal trans- 

 mission within it, its large peripheral surface and the 

 area of its contact with the cambium and medullary 

 ravs seem to suit it for the transmission of organic 

 substances in a radial direction. In this connection 

 the medullary rays may have the function of discharg- 

 ing into, and extracting from, the tracheae organic sub- 

 stances which are transmitted to and from them by 

 iie bast. The observations made bv Atkins and one 

 if us that the concentration of carbohydrates in the 

 txinducting tracts often diminishes from below up- 

 wards suggests that these substances mav be extracted 

 NO. 2730, VOL. 109] 



from the transpiration current in its upward move- 

 ment (H. H. Di.\on and W. R. G. Atkins, Notes 

 from the Botanical School of Trinity College, Dublin, 

 vol. 2, pp. 335 et seq.). The presence of starch in the 

 medullary-ray cells in many plants at all times of the 

 jear suggests that the carbohydrates are fi.xed in these 

 cells as starch. Solution by enzymes of this starch in 

 response to an upward or a downward movement of 

 the water in the tracheae would provide a mechanism 

 for the upward or downward transport of these sub- 

 / stances in the transpiration stream. 



It is hoped by experiments which are now in pro- 

 gress to throw some further light on this fundamental 

 problem of plant physiology. 



Henry H. Dixon. 



Nigel G. Ball. 

 - Trinity College, Dublin. 



Lunar Periodicity in Reproduction. 



It is a common belief in many fish-markets around 

 the Mediterranean and in other parts of the world 

 that the amount of edible matter in sea-urchins and 

 certain other invertebrates varies with the phases of 

 the moon. The animals are said to be " full " when 

 the moon is full and " empty " at new moon. This 

 belief was recorded by Aristotle, Pliny, and other 

 classical writers, and w^as stated by them to apply 

 not only to sea-urchins, but also to oysters and other 

 molluscs. 



During the summers of 1920 and 192 1 I made syste- 

 matic examinations of the gonads of an Echinoid 

 (Diadema setasum) at Suez with the object of testing 

 the truth of the popular statement. I found the latter 

 to be founded on fact to a surprising degree. There 

 is a periodic reproductive cycle in this species^ of 

 Echinoid which is correlated with the lunar period, 

 the genital products being discharged into the sea 

 at about each full moon during the breeding 

 season. An examination of the testes and ovaries of 

 a number of individuals between the first quarter and 

 full moon shows the majority to be swollen and 

 full of mature spermatozoa or eggs, while a lesser 

 number are " spent," i.e. show evidence of having 

 lately extruded their genital products. A week later 

 the relative proportions are reversed. Some in- 

 dividuals have gonads still full of spermatozoa or 

 eggs, but most are now " spent." Between the third 

 quarter and the new moon all gonads are shrunken in 

 size and contain nothing but developing spermatocytes 

 or oocytes. From now onwards until the first quarter 

 of the next moon these cells show progressive stages 

 in development into spermatozoa and eggs which are 

 to be spawned at about the time of full moon. 

 This lunar cycle is repeated throughout the breeding 

 season. 



In seeking a causal connection between the 

 reproductive rhvthm and the lunar month an influence 

 of the tides first suggests itself. But whereas there 

 is a single reproductive cycle in each lunation, there 

 are two spring and neap tidal periods, i.e. a double 

 cvcle. However, during the summer months at Suez 

 the new moon spring tides have a greater range than 

 those of the full moon, so that the maximum tidal 

 range is attained onlv once during each lunar month. 

 The higher and lower water at the new moon spring 

 tides might conceivably react on the Echinoids by the 

 different hydrostatic pressure (affecting, e.g., the ten- 

 sion of dissolved gases) or bv causing the animals to be 

 at a greater or less distance than usual from the 

 source of oxygen or of light. But the average excess 

 tidal range at new moon spring tides over that at full 

 moon spring tides during the period studied was only 

 58 cm. This small difi'erence could scarcely aff'ect the 



