NUTRITION 287 



c c c c 



I I I I 



A o B B A A o B B A 



D o E D o E 



F F 



It will be seen that one or the other carbon atom is supposed to be turned by 180 

 around the longitudinal axis. See further description by van't Hoff (1901. 2tes 

 Heft, p. 110). 



How far all these isomers are capable of existence in a given case is a matter for experiment, 

 but the optical rotations of the four different substances may be understood better if we call 

 the effect of the one carbon atom, A, and that of the other, B. Then we have the four following 

 results : 



+ A +B, -A +B, +A -B, -A -B. 



Out of these relative isomers, it is to be presumed that one will be the most stable and that 

 the different relative positions of the various groups will entail different properties. We may 

 take lactic acid and brucine as A and B, for the sake of illustration, so that there are : 

 rf-brucine d-lactate, rf-brucine /-lactate, /-brucine d-lactate, and /-brucine Mactate. 



These facts are of importance in connection with the variety of processes in 

 the living organism. It appears that there is evidence of the existence of malic 

 acids in greater number than usually supposed. Erlenmeyer shows further 

 (p. 447) that, if the d- and I- forms of a substance are not absolutely fixed 

 " point systems," but convertible by external forces into relative isomers by rotation 

 around an axis of combination, then a force acting in the same direction on the 

 two mirror-images will have a different effect on them, so that they may be 

 converted into modifications which are no longer mere mirror-images of one 

 another, and can therefore be separated by means which do not involve the use 

 of optically active reagents. 



The experimental evidence is as follows : If a solution of rf-/-asparagine (inactive) be 

 boiled for twenty minutes, /-asparagine separates out first on cooling. If d- and /-asparagine 

 are dissolved cold in the same solution, no separation is possible, since their solubilities are 

 identical. One of the two is altered by heating, since the relative solubility is changed. By 

 further fractionation, Erlenmeyer succeeded in preparing specimens of d- and /-asparagine in 

 nearly pure condition. It appears to be impossible to predict whether d- or /-asparagine will 

 be obtained in any particular experiment. Further details and also the formation of two 

 different copper salts will be found described in the original paper. The same phenomena 

 were found to exist in the case of the tartrates. 



Should these results be confirmed, it is clear that we have a possibility of the 

 production in the plant of optically active substances from racemic mixtures. One 

 of the "relative isomers" would probably be more easily oxidised for energy 

 purposes, leaving in excess the other mirror-image, which had not been changed. 

 Having once obtained asymmetrical compounds, further production is comparatively 

 simple, as we have seen. 



It will be noticed that the change from one relative isomer to the other is much 

 more readily brought about than that from one optical isomer to the other. The 

 asparagine which has been changed by boiling goes back into its original form 

 when redissolved in cold water. 



An important point is that the method is not a general one, and it is very 

 suggestive that these two substances, asparagine and tartaric acid, which have 

 been shown to manifest the property, and especially asparagine, are of such 

 common occurrence in plants that it is by no means unlikely that optical activity 

 commenced in this way. The work of Horace Brown (1906), on the part played 

 by asparagine in the protein metabolism of the plant, is also to the point here. 



GROWTH 7^ VITRO 



The experiments of Ross Harrison and of Carrel have been referred to 

 previously (page 23). A few remarks with regard to the chemistry of the process 

 are of interest in the present connection. It is plain that in these experiments, 

 granting that new tissue was actually formed, which appears to be satisfactorily 

 shown by the presence of dividing nuclei, the proteins of the blood plasma, used as 



