Sec. 27.12] TRACE ELEMENTS ESSENTIAL TO ANIMALS AND PLANTS 529 



is an important but not the sole pathway for the passage of cobalt from the 

 body into the intestinal tract [Co7,9]. 



Recently the long-lived species Co 60 has come into use. It has been 

 employed in studies on cobalt distribution, particularly, in the bone marrow 

 [Co8]. 



27.8. Molybdenum. For molybdenum, an element required by certain 

 higher plants, there are two known radioactive species of convenient half-life: 

 Mo" (67-hr half-life), which suffers, however, by being the parent of radio- 

 active Te"; and Mo 93 (6.7-hr half-life). In addition there are a number of 

 stable species that might prove of value. 



Mo" has recently been used in preliminary tracer work in animals [Mol] 

 and plants [Mo2]. Mo 93 has also been used in plant studies [Mo2]. 



27.9. Boron. Boron, which is known to be an essential micronutrient of 

 higher plants and some microorganisms, has not been studied by tracer 

 methods. The only possible tracer isotope known is the stable species 

 B 10 , which constitutes 18.4 per cent of the element. This species is now 

 available in nearly pure form. 



Work has, however, been done with boron in the study of the effect of the 

 selective ionization produced by bombarding this element concentrated in 

 tissues [Bl-3] with slow neutrons. These studies have suggested that 

 the splitting of B 10 under such conditions may possibly find therapeutic 

 application. 



27.10. Aluminum. Aluminum, which is known to be essential to certain 

 plants and protozoans, has also not been studied by tracer techniques. The 

 only available species would seem to be Al 28 and Al 29 with half-lives of only 

 2.4 and 6.7 min, respectively. At best these offer very limited potentialities. 



27.11. Silicon. Silicon, which is also known to be essential in the nutrition 

 of certain plants and protozoa and which may also be a necessary micro- 

 nutrient in higher animals, is another element not yet studied by biological 

 tracing methods. Possibly useful species are the two rarer stable species 

 Si 29 and Si 30 and the radioactive species Si 31 (170-min half-life). 



27.12. Vanadium. Vanadium is an essential part of a metalloporphyrin 

 respiratory pigment in certain lower chordates. Its metabolism could 

 readily be studied with the radioactive isotope V 48 (16-day half-life) and 

 possibly with V 49 (600-day half-life). 



