Sept, i, 1921 
Growth in Branches of Young Pear Trees 
871 
The most casual observer is aware, however, that the lateral shoots 
arising from an older vertical shoot are by no means equal in length. 
The longest lateral is usually produced at the apex of the mother shoot, 
and each successive lateral is shorter than the preceding. 
This characteristically greater growth of new shoots at the distal end 
of an upright branch, especially after pruning, is a well-recognized 
condition. An adequate understanding of its causes is greatly to be 
desired. Many of the statements on the subject to be found in botanical 
literature are vague, and others are mystifying. For example, the 
mere statement that “the available food materials of the stem are 
principally devoted to the development of the apical bud’’ raises more 
questions than it answers. The statement that the growth on an up- 
right branch is regulated by polarity falls into the same category. In 
the sense that “polarity” has been applied to the formation of shoots 
and roots on cuttings this idea would be inapplicable to the factors 
influencing fruit-spur formation, since, in its implication, “polarity” 
involves the idea of two mutually exclusive, opposite qualities—for 
example, the opposite ends of a magnetized bar of metal. The idea of 
an axial gradient as used by Child (2) is a more exact designation 
of the phenomenon but does not account for the production of the 
gradient. If it be solely a question of the distribution of nutrient 
materials in the parent branch it is difficult to understand why the 
growing points at the distal extremity of the branch should acquire such 
quantities as would result in the production of larger shoots, while the 
growing points near the proximal extremity usually fail altogether to 
develop into shoots in the first season. 
It seems logical to assume that the type of development observed 
depends not upon the distribution of nutrient materials in the parent 
branch but upon the distribution of some substance which is antagonistic 
to growth. This substance may be formed in the distal region of the 
shoot and migrate toward the base of the shoot. Certain evi¬ 
dence on this point has been already presented (11). We may, 
therefore, assume that the amputation of a portion of the mother shoot- 
removing the apical bud and a number of adjacent subapical buds_ 
removed the most active centers in which some sort of a growth- 
inhibiting substance was produced. The effect of this operation was to 
remove an obstacle which impeded the development of subapical buds 
into shoots. This is an aspect of the theory of chalones which are 
recognized as factors in the growth of the animal body and which have 
been discussed in relation to plant growth by Loeb (5) and Reed 
and Halma (ro, 11). On the basis of this assumption it would appear 
that the buds immediately below the point of amputation grow first 
because they are the first to be freed from the inhibiting substance. 
As soon as the new shoots begin to grow they begin to produce more 
