Urea as sole source of nitrogen for plant growth

Summary Spirodela oligorrhiza grown in sterile culture was able to use urea as sole source of nitrogen but only when the pH of the culture medium was below 4.3. Plants inoculated into urea media at pH 6.4 initially made little growth and became nitrogen-deficient in appearance and composition although they contained about 100 grams of urea per gram fresh weight of tissue. After a period the pH of the medium usually fell below 4.3 and growth commenced. Growth with other compounds, e.g. ammonium, nitrate or allantoin, as sources of nitrogen was not similarly affected by the pH of the culture medium.Urease activity could always be detected in the tissues of Spirodela oligorrhiza growing on urea. Plants with little or no urease activity soon developed significant activity when inoculated into urea media at pH 4.0. When the pH of the medium was higher there was no increase in urease activity and no growth ensued. Plants growing on urea possessed an activity of about 50 milliunits per gram fresh weight of tissue, but if the pH of the medium fell to 3.5 or lower, the activity present rose to 10 times this level.Urease activity also appeared, in the absence of supplied urea, as plants became increasingly nitrogen-deficient.     

Mutual diffusional interference between adjacent stomata of a leaf

The mutual diffusional interference between adjacent stomata in laminar flow over a leaf is shown to play a decisive role in determining overall transpiration. The magnitude of this interference varies with the interaction of the vapor diffusional shells forming above each stoma and the air flow over the leaf. The interference decreases with increasing incident radiation and wind velocity. The effect of interference on the stomatal resistance to diffusion plays a major role in the overall variations in transpiration.     

Isolation of deoxyribonucleic acid from mammalian tissues

1. DNA has been isolated from different mammalian tissues. The DNA preparations were free from RNA, protein and polysaccharides and have a similar range of sedimentation coefficients (approx. 24s). 2. Protein was removed by a two-stage extraction with a phenol-cresol mixture by using a detergent with 4-aminosalicylate in the first stage and sodium chloride in the second. 3. Polysaccharides remained in solution when DNA was precipitated with 2-butoxyethanol in the presence of 0.5m-sodium chloride and 1.5m-sodium benzoate. 4. Ribosomal RNA was removed by precipitation in the presence of 3m-sodium chloride at 0 degrees , when DNA remained soluble.