Sept, i, 1921 
Growth in Branches of Young Pear Trees 
869 
DISCUSSION 
It remains to see how the data presented above will contribute to an 
understanding of the growth process in trees, especially in relation to 
the characteristic formation of vegetative and fruit-bearing shoots. It 
hardly requires further discussion to show that the new shoots on the 
pear tree arise in such a perfectly definite and orderly fashion as to sug¬ 
gest that their growth and distribution is governed by definite causal 
factors. The problem is to discover, if possible, these causal factors 
and their functions. The discussion will adhere in a general way to 
the arrangement of data presented in the preceding pages. 
The total amount of new shoot material produced on pruned and un¬ 
pruned mother shoots ought to show what result the stimulus of pruning 
may produce, and to give a basis for discussing the nature of accelerated 
growth. 
A comparison of the amount of new growth produced in the two cases 
shows that the branch system is larger at the end of the season than 
would be required if the growth response were merely a means of re¬ 
storing what had been removed by pruning. In other words, the ampu¬ 
tation of part of the mother shoot has resulted in producing more new 
growth on what remained of that particular shoot than would have been 
produced if it had remained uncut. The problem is not, why such a 
shoot produces new laterals, but why it produces a mass of new 
laterals about 65 per cent greater than it would if left unpruned. Since 
this problem is fundamental to the theory and practice of tree pruning, 
it may be profitable to extend the inquiry as far as possible. 
The simplest conception of growth which we seem warranted at pres¬ 
ent in holding is that it is the result of some sort of a catalyst acting 
upon substances acquired by, or produced in, the organism. In the 
absence of contrary evidence we must believe that all buds and other 
meristematic tissues are supplied with either the active or the potential 
catalyst. The ratio between the growth of a particular organ and the 
growth of the rest of the tree must in some way depend upon the relative 
activity of the catalyst of these organs. 
In connection with these suggestions reference may be made to a paper 
on the growth of pruned and unpruned branches of the apricot tree (9). 
As a result of the pruning the final mean length of new shoots was 210 
cm., while it was 94 cm. on the unpruned neighboring trees. The equa¬ 
tion for the growth of these apricot shoots was that of a monomolecular 
chemical reaction, 
x = a (i — e~ kt ), 
in which x represents the length of the shoot at time /, a is the final length 
of the shoot, k is a constant, and e is the base of the natural logarithms. 
Having the values of a and k t it is possible to compute the length of the 
shoots for any value of t in the growing season. For these apricot shoots 
