CHAPTER XII 



THE INHIBITORY ACTION OF APICAL LEAVES ON THE 

 FORMATION OF SHOOTS IN THE LOWER PART OF 



THE STEM 



1. There exists a group of phenomena which allows us to put 

 the hypothesis of polarity in regeneration developed in the pre- 

 ceding chapters to a further test. These phenomena are con- 

 cerned with the inhibition of shoot formation in a stem by leaves 

 at the apex of the stem. This inhibition is observed in the 

 stems of young plants of BryophijUum (less than one year old) 

 when suspended vertically and upright, either in moist air or with 

 their bases in water. When the stems of old plants are used or 

 when the stems are suspended horizontally, different results are 

 obtained which will be discussed in their proper place. 



In order to understand these phenomena attention should 

 once more be called to the arrangement for the anlagen of 

 shoots in stems of Bryophylluin. In the axil of each leaf of a 

 stem there exists one bud capable of giving rise to a shoot. Each 

 node of a plant has 2 leaves in opposite position, and the axis 

 connecting the 2 axillary buds in one node is always at right 

 angles with the axis connecting the two buds of the next node 

 (Fig. 69). Thus the line connecting the two buds at node 2 

 (Fig. 69) is at right angles with the line connecting the two buds 

 in node 1 as well as in node 3, etc. The lower leaves on a stem 

 fall off in time, leaving their axillary buds exposed. 



In Fig. 70 the path of the sap from an apical leaf is indicated by 

 shading. The sap from one leaf goes through the anlagen for 

 both shoots in the first and third node below the leaf, and through 

 the anlagen of one shoot in the second and fourth node below the 

 leaf; namely, on the side of the apical leaf. The buds in the 

 second and fourth node below the apical leaf and on the opposite 

 side of the stem alone are not in the path of the descending sap 

 from the leaf and we shall find that they alone can grow into 

 shoots. 



93 



