376 GROWTH 



than another, or may have a higher surface tension and so on. 

 Although it has been pointed out that the presence of external 

 resistance acts as a stimulus to growth, it has also been said that 

 internal resistance arrests growth. 



Bohn propounds the following four laws relating growth and 

 form in plants, and they may be applied to animals with some 

 measure of justification : 



(1) Law of Vectors. A vector in distinction from a scalar 

 phenomenon is one representable graphically by a line of known 

 direction and definite length, i.e. a conception of a change of 

 magnitude with time. " The principal forces of growth are 

 directed along axes offering a geometrical disposition." 



(2) Law of Depolarisation. " When growth becomes exag- 

 gerated in a certain direction, a force is developed in the living 

 organism which tends to oppose the growth, " i.e. Errera's Rule, 

 q.v., or the ordinary law of balanced reaction. 



(3) Law of Axial Repulsions. " When secondary axes are 

 borne on the principal axis of a plant or animal, a reciprocal 

 repulsive force is developed between the principal axis and each 

 secondary axis." 



(4) Law of Compensation. When an axis branches in one plane 

 there is a tendency for the re-establishment of the destroyed 

 bilateral symmetry." 



To summarise : 



1. Growth is a balanced reaction having the mature organism 

 for an end point. 



2. The organism grows at a definite rate which is, at any 

 moment, proportional to the amount of growth yet to be made. 



3. The rate of growth is not uniform throughout but is specific 

 for each epoch of life. 



4. The growth in each epoch proceeds at a rate corresponding 

 to an autocatalytic reaction. 



5. Various factors which influence chemical and physico- 

 chemical reactions, influence the rate of growth. 



6. If the rate of growth is arrested in any epoch, the length of 

 time spent in that cycle is prolonged so that the amount of 

 growth characteristic of that epoch is accomplished. 



7. Form is a function of growth. 



