Under these conditions, the mean growth rates (over a 5 week period) of Pc, Hc, and Tc were 13.4 ± 0.7, 6.9 ± 0.5, and 19.4 ± 0.9 g tissue/g inoculum, respectively.Tissues of Zea mays L. cv. Burpee Snowcross were also employed in this investigation. Surface-sterilized and soaked seeds were allowed to germinate in the dark at 25 C, and roots were collected 48 h after the time of planting.Extraction, Purification, and Hydrolysis of RNA. Tissues of N. tabacum and of Z. mays were harvested, weighed, and frozen immediately. Tissues were harvested 5 weeks after introduction into culture. The method described by Short and Torrey (16) was used to extract RNA from these tissues, and the procedure of von Ehrenstein (20) was employed for the purification of tRNA. The collected tRNA precipitate was dissolved in 50 ml distilled H20 (2 C) over a 48-h period, lyophilized, and weighed or estimated spectrophotometrically. Assuming the equivalence of 20 A units at 260 nm and a concentration of 1 mg/ml (10,16,19), the tRNA recovery from these tissues was approximately 0.08 mg RNA/g tissue (fresh weight). These results were similar to those of Chen and Hall (5)