METABOLISM OF THE B VITAMINS 337 



are capable of binding the B vitamins, either to create structural enzyme- 

 coenzyme forms or insoluble storage forms, our knowledge of the mech- 

 anism of catalysis would suggest that they also would be able to liberate 

 these forms. If, as suggested (p. 316), biotin functions as a hormone in 

 plants, flowing downward from the tip, then its solubilizing release from 

 the bound form in an auxin-like fashion (p. 37) is of major importance 

 in plant differentiation. 1 - 225 In the case of many Thallophytes, digestion 

 as we know it may frequently occur outside the plant. Although the bac- 

 teria particularly are well known for their ability to liberate vitamins 

 from combination, in some cases, as in the avidin-biotin complex, even 

 bacteria may be incapable of digesting the bound form. There is a more 

 detailed discussion of enzymatic liberation of B vitamins earlier in this 

 volume (Chap. Ill A). 



It is generally assumed that water-soluble forms of the B vitamins 

 may diffuse readily in and out of plant cells, but that bound forms, par- 

 ticularly protein-bound forms, do not. These assumptions are frequently 

 unjustified, and are based largely on analogy with questionable data in 

 the animal kingdom. Since there is little evidence bearing directly on the 

 subject in plant cells and tissues, it seems pertinent at least to point out 

 that the diffusion across a cell membrane or tissue barrier may involve 

 intermediate formation, and is most frequently a selective process not 

 involving simple diffusion in the strictly physical sense. The active 

 absorption of thiamine by live yeast in the animal intestine is particularly 

 significant in this regard. Further, some bound forms, even proteinaceous 

 ones, may be quite capable of diffusion across "semipermeable" mem- 

 branes. Further extended studies will therefore be required to determine 

 the nature of this process in plants. 



Distribution, Catabolism, and Excretion in Plants. The salient facts 

 with regard to the distribution of the B vitamins in plants have been 

 considered in an earlier section (Chap. II A). Thiamine, vitamin B,3, 

 niacin, pantothenic acid, and biotin seem largely concentrated in the seeds, 

 while riboflavin, inositol, and folic acid are most concentrated in the 

 leaves. Without dwelling unduly upon this latter fact, and without ques- 

 tioning the importance of all B vitamins to the photosynthetic process, 

 it seems of sufficient interest to note that inositol occurs in leaves largely 

 as the hexaphosphate, 2 and may function in this case as a phosphate 

 storage form; that riboflavin has been implicated in phototropism in 

 plants, 3 a phenomenon that is closely integrated with photosynthesis; and 

 that folic acid may well have a very special role in the photosynthetic 

 process. 



Strangely, little is known concerning the role of the B vitamins in 

 photosynthesis, and in one recent monograph on photosynthesis 4 no 

 B vitamin is even so much as mentioned in the index. It is known that 



