In studying venation, and in tracing the course of vascular bundles in large ovules and in other organs, it is often an advantage to use a stain. If a stem of Impatiens be cut under water, and the cut surface be then placed in a dilute aqueous solution of eosin, the eosin will rise in the vessels, making them very prominent. The outer bundles of the large ovules of cycads are very easily studied by this method. The inner bundles also may be seen by opening the seed and removing the endosperm.
If such preparations could only be cleared, they would be still more valuable, but the effect is due to the presence of the staining fluid in the vessels, and any subsequent treatment diffuses or destroys the stain. Perhaps a little experimenting might obviate the difficulty.
Some stains will stain living structures. Cyanin, methyl blue, and Bismarck brown have been recommended for this purpose. The solutions should be very dilute, not stronger than 1:10,000 or 1:500,000. The solutions should be very slightly alkaline, never acid. It is claimed that such solutions never stain the nucleus, and that if the nucleus stains at all, it is an indication that death is taking place.
Campbell succeeded in staining the living nuclei in the stamen hairs of Tradescantia by using dilute solutions of dahlia and of methyl violet (0.001 to 0.002 per cent in water). Dividing nuclei were stained.
For determining the stage of development of fresh material it is often necessary to use a stain. For this purpose stronger stains may be used, since it is unimportant whether the tissue is killed or not. An aqueous solution of methyl green or eosin can be recommended. With 1 per cent solutions, diluted one-half with water, mitotic figures can be recognized with ease.
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