Mode, Site, and Time of Initiation of Hypocotyledonary Bud Primordia in Linum usitatissimum L.

Abstract

1. Transverse division of an epidermal cell is the first identifiable stage in initiation of a hypocotyledonary bud primordium in L. usitatissimum L. 2. Generally, a second transverse division in one or both of the daughter cells, but not infrequently either a radial or a tangential division, is the second stage. 3. These divisions are not preceded by grossly detectable increase in size of the cells involved, nor is there marked increase of the cytoplasmic contents of these cells prior to or immediately following these divisions. 4. These events begin in epidermal cells of the lower half of the hypocotyl between the 4th and 5th days of the developing seedling. Development of primordia of the lower half of the hypocotyl into buds, and of these buds into shoots, rarely occurs in the normal, non-decapitated hypotocyl. 5. Upon decapitation of the hypocotyl, one to many of the bud primordia of the lower half of the hypocotyl may develop into buds, and one or several of these may develop into shoots. 6. The epidermal cells of the upper half of the hypocotyl also are capable of initiating bud-primordium formation but do not do so unless the hypocotyl has been decapitated. Following decapitation between the 3d and 15th days after emergence of the seedling, a great number of primordia may be initiated by these epidermal cells. Many of these primordia may develop into buds, and one (occasionally several) of these buds may develop into a shoot. After a shoot from either the lower or the upper half of the hypocotyl has established dominance, the development of the other shoots ceases or proceeds very slowly. 7. The epidermal cells apparently must be in a definite stage of development for initiation of the chain of divisions which characterizes bud-primordium formation. This stage apparently is realized while or after the epidermal cells are in the final stage of elongation and persists for a time after this elongation has ceased. It is first attained in the zone of general elongation in the lower half of the hypocotyl and proceeds upward as this zone moves up along the hypocotyl. 8. Presence of the cotyledons and epicotyl does not inhibit initiation of bud primordia in the lower half of the hypocotyl but does inhibit it in the upper half. Generally, the presence of these organs and, later, of the epicotyl alone inhibits development of the bud primordia of the lower half of the hypocotyl into buds and of the occasional bud into a shoot. 9. Generally, if hypocotyls are decapitated after the 20th-30th days after emergence, no bud primordia are formed in the upper half of the hypocotyl, but some of the primordia present in the lower half may develop into buds, and, of these, one or rarely two may develop into a shoot.