HISTOLOGICAL STUDIES ON THE COLEOPTILE. I. TISSUE AND CELL TYPES IN THE COLEOPTILE TIP

Abstract

The uppermost 1‐4 mm of 25‐mm coleoptiles of oats and wheat have been studied at the optical microscope level, using newer histological methods and sections 1‐4 μ thick. The outer epidermal wall, which shows very fine wrinkling, is continuous with the thinner wall of the inner epidermis through the pore. The cells of both epidermal layers have acidophilic cytoplasm with long transvacuolar strands. Both inner and outer epidermis have stomata, those of the outer epidermis having kidney‐shaped guard cells like those of dicotyledons. The guard‐cell walls are lignified in their inner layers only and are thinly cuticularized. In the vascular bundles the sieve tubes terminate apically about 250 μ below the end of the xylem; the xylem in turn terminates about 400 μ below the extreme apex. A number of clearly undifferentiated cells, with highly basophilic cytoplasm and many mitochondria, separate the xylem elements from the inner epidermis. Towards the outer epidermis there are a few sieve elements, each of which is associated with a special cell having an elongated nucleus supported on fine cytoplasmic strands. The parenchyma of both the tip and the shaft of the coleoptile are generally interpenetrated by air‐spaces, but where they are adjacent to the inner epidermis there is heavy interposition of readily stained intercellular material, especially in Triticum. Plastids are widely distributed throughout the tissue, but their greening in light takes place preferentially towards the phloem side of the vascular bundles. The observations are discussed in reference to earlier literature and with regard to the function of the coleoptile as a protecting and guiding organ for the shoot system of the seedling.