The Application of Freeze-Dry Methods to Plant Material

The freeze-dry method of preparing tissue for histological observation has been applied to a large variety of plant material with good results. This method involves freezing the tissue rapidly and dehydrating at a temperature below -30°C. under vacuum with a desiccant. The tissue, when dry, is infiltrated with paraffin under vacuum. The apparatus uses a liquid nitrogen-cooled condenser as the desiccant and the tissue is infiltrated with paraffin while in the original vacuum. Using root, stem, leaf, and reproductive tissue of common experimental species such as Vicia faba, Zea mays, Allium cepa, Lillium longiflorium, Pisum sativum and Phaseolus vulgaris, the effect of thickness on drying time was studied. It was found that there was greater difference between types of tissue than between species so that regardless of species, leaf tissue was easiest and root tips the most difficult to freeze and dry. Leaf segments required 1-2 days to dry while root material required 6 days or more. In all tissue except leaf, 1-2 mm. segments were optimal for freezing and drying although 4 mm. segments could occasionally be used.