Experimental Studies on Leafy Spurge (Euphorbia Esula L.). I. Ontogeny and Distribution of Buds and Shoots on the Hypocotyl

Abstract

Adventitious buds of the additional type develop on the hypocotyls of seedlings of leafy spurge. Most additional buds are produced in young seedlings, and their pattern of distribution on the hypocotyl is about the same in decapitated and nondecapitated seedlings. The proximal part of the hypocotyl contains more buds than the middle or distal parts. As the seedlings age, more shoots are developed in the proximal region than in the middle or distal parts of the hypocotyl. When young seedlings are decapitated, shoots tend to develop more in the middle part than in the proximal part. In older seedlings, decapitation results in the production of more shoots in the proximal region than in the middle part of the hypocotyl, a feature comparable to nondecapitated seedlings. Apical control of shoot development on the hypocotyl diminishes gradually as the seedlings age. The hypocotyledonary buds are exogenous or endogenous in origin. The exogenous buds are distributed on most of the hypocotyl, and the endogenous ones are confined to the extreme proximal part near the collet. The initials of the exogenous type arise in the subepidermal cells of the cortex and proliferate to form buds, some of which develop into shoots. The endogenous buds arise in the pericyclic tissues of the stele, and they develop into shoots which are eventually more successful in their growth than the exogenously developed shoots.