Abstract
When a piece is cut out of the stem of a plant (Bryophyllum calycinum) the most apical buds will grow out into shoots, while roots develop chiefly but not exclusively at the basal end. The writer suggests the following explanation of this phenomenon of polarity. In the normal stem the growing region at the apex as well as the leaves send out special inhibitory substances toward the base of the plant which prevent the growth of the more basally situated dormant buds capable of giving rise to shoots. When a piece is cut out from the stem these inhibitory substances continue to flow in the piece toward the base and the most apical node will be the first one sufficiently free from these inhibitory substances and hence the two dormant buds situated at this node will grow out first. As soon as they grow out they produce and also send out inhibitory substances toward the base, thereby preventing the more basally situated buds from growing out into shoots.
A brief outline of the experiments supporting this hypothesis has been published in Science. 2
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