Histological and Regenerative Studies on the Flax Seedling

Abstract

1. Germination of the seed and development of the seedling of Linum usitatissimum are described. 2. The primary root is diarch. The growing point has clearly defined histogens. The plerome, periblem, and calyptrogen give rise to the stele, cortex, and root cap respectively. Laterally to the calyptrogen, the dermatogen produces the epidermis by anticlinal divisions. 3. In the development of the vascular elements of the root, two primary phloem ducts are first differentiated by elongation and breakdown of a single row of cells lying next to the pericycle and alternate with the protoxylem points which are differentiated later. 4. Lateral roots are initiated by regular divisions of some of the cells of the pericycle. One tangential division of these cells of the pericycle forms two layers which in turn divide simultaneously and form four layers of cells. The outer layer later differentiates as the calyptrogen and dermatogen, the next as the periblem, and the innermost two as the plerome. 5. In formation of lateral roots the endodermis becomes active and forms a single layer of meristematic cells overlying the root cap. 6. A simple type of transition from root to cotyledons occurs in the hypocotyl and base of the cotyledons. The primary vascular system of the root, hypocotyl, and cotyledons forms a system largely independent of the subsequent epicotyledonary development. The principal vascular connection of the epicotyl and hypocotyl is by secondary tissues. 7. In the upper region of the hypocotyl most of the primary xylem is broken down by elongation, and is for the most part resorbed by intervening parenchymatous cells. 8. Development of the epicotyl and foliage leaves is described. All vascular bundles of the epicotyl are collateral foliar bundles which do not anastomose but end in the parenchyma between other vascular bundles. The only vascular connection between the primary vascular bundles of the stele is by a continuous vascular cylinder of secondary tissues which is formed by the activity of fascicular and interfascicular cambium. 9. The basal part of seedlings severed at the middle region of the hypocotyl regenerated five to twenty buds, of which usually only one continued development and became a shoot. Regenerated buds originated by successive division of epidermal cells. Vascular connection between the primary stele of the hypocotyl and the new shoot was initiated by renewed cell activity of cortical parenchyma, endodermis, pericycle, phloem parenchyma, and cambium, in the order named. Tracheids of the vascular system were first differentiated in the leaves of the new bud, and then differentiation occurred progressively inward to the stele of the hypocotyl until continuity was established between the tissues of the new shoot and the stele of the hypocotyl. 10. Seedlings severed in the middle region of the hypocotyl and the upper parts used as cuttings set in moist soil or placed in moist petri dishes produced adventitious roots from the hypocotyl. These roots originated from parenchymatous tissue which in the same root often involved pericycle, phloem, and pith. The endodermis of the hypocotyl became active and produced a single layer of cells overlying the tip of the developing root. 11. Cotyledons cut from seedlings and placed on moist soil produced adventitious roots and lived for more than two months. These roots originated from a layer of parenchymatous cells on the adaxial side of the larger veins. In no case did these cotyledons produce shoots.