148 RADIOISOTOPES IN BIOLOGY AND AGRICULTURE 



tail vein or by cutting the tail. To obtain blood at the time of sacrifice, 

 the thorax can be opened and the heart exposed for direct puncture. 



With the guinea pig, from 5 to 15 ml of blood may be obtained by heart 

 stab as described above for the rat. For collection at sacrifice the animal 

 is anesthetized or stunned, and the large vessels of the neck are exposed 

 by incision and then severed. 



Up to 20 ml of blood may be obtained from a 5-lb rabbit by heart stab. 

 About 2 or 3 ml can be collected by free bleeding from a marginal ear vein 

 after puncture with a large needle. It helps if the ear is fiipped with the 

 hand and treated with xylene and alcohol before puncture. 



Blood can be collected from dogs, swine, sheep, cattle, and horses by the 

 same methods as employed for injection. One advantage in the use of 

 farm animals in experimentation is that relatively large quantities of 

 blood can be drawn frequently during the course of the study. In many 

 studies, blood samples must be taken frequently and in large numbers, in 

 which case considerable injury to the vein will result if a puncture is made 

 for each sample. Under these conditions there is considerable advantage 

 in the use of a catheter that is inserted as previously described. 



PROCEDURES WITH PLANTS AND SOILS 



Plant-nutrition research with radioisotopes may range from laboratory 

 studies with excised roots through greenhouse work with potted plants to 

 full-scale field applications of labeled fertilizers. In general, conventional 

 techniques are adequate, and mention will be made only of some typical 

 adaptations for work with radioactive materials. 



Closed System for C^^ Studies. It is necessary to grow plants in a 

 closed system in which the atmosphere can be altered for the investigation 

 of carbon fixation or for the biosynthesis of carbon-labeled compounds. 

 Simple methods have been described by Burris et al. (36), and their pro- 

 cedure as reported below represents the basis of other, more elaborate 

 arrangements. 



Figure 4-22 (36) shows the simple chamber that permits the internal 

 generation of CO2. The plant is grown under ordinary conditions in 

 a porous pot and is placed in a glass jar when ready for experiment. 

 Another glass jar of equal diameter is drilled near the bottom to furnish 

 a hole for the generating test tube. This jar is placed over the first one 

 and fastened to it with adhesive tape, after which the joint is painted with 

 a 50 : 50 beeswax-rosin mixture to make it airtight. A hole is blown in the 

 side near the bottom of the test tube to be used for the generation of the 

 CO2. The BaCi^Os is placed in the test tube, which is then closed at the 

 top with a serum- vial rubber stopper and sealed into the upper glass jar 

 with beeswax-rosin. If desired, a small vial of indicator solution can be 



