338 



PHYSIOLOGICAL BASIS OF POLARITY. 



region of the piece will be the first to become sufficiently free from 

 these substances, the buds situated at this end will be the first to 

 grow out into shoots. As soon as this happens the new shoots will in 

 their turn send out inhibitory substances in a basal direction in the 

 stem, thereby preventing the growth of the more basally situated 

 buds. In this paper we shall present some of the qualitative evidence 

 for the inhibitory effect of a leaf upon shoot formation, leaving the 

 discussion of experiments of a more quantitative character for a 

 future paper. 



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Fig. 1. Diagram showing arrangement of shoot buds in the stem of Bryo- 

 phylhim calycinum. The line connecting the two buds in one node is always at 

 right angles to the line connecting two buds in the next node. 



We shall deal chiefly with the regeneration of shoots in the stem of 

 Bryophyllum calycinum, which can proceed only from definitely lo- 

 cated buds. In the axil of each leaf of a stem there exists one bud 

 capable of giving rise to a shoot, which, however, never does so unless 

 the plant is mutilated. Each node of a plant has two leaves in 

 opposite position, and the axis connecting the two axillary buds in 

 one node is always at right angles with the axis connecting the two 

 buds of the next node (Fig. 1). Thus the line connecting the two 

 buds at Node 2 (Fig. 1) is at right angles with the line connecting the 

 two buds in Nodes 1 and 3, etc. The lower leaves in a stem fall off 

 in time, leaving their axillary buds exposed. No other element of the 

 stem except the two buds in each node is capable of growing into shoots. 



