NORMAL AND PATHOLOGICAL GROWTH 145 



Hooker's 1 study of the cycles of growth, rest, and fruition in 

 plants is a most notable recent contribution to this field. He has 

 found that the stages of rapid growth in fruit trees, with the 

 extensive development of the chlorophyllic and photogenic ap- 

 paratus which produces and stores carbohydrates, slows down in 

 activity as carbohydrate accumulates in the storage reservoirs. In 

 other words the development and presence of the end products 

 of the activity of the photogenic apparatus are inhibitive and 

 restraining to these processes. This may even go so far as to lead 

 to the destruction of the photogenic apparatus itself. The pres- 

 ence of an excess of stored carbohydrates, on the other hand, is 

 favorable to the development of the carbohydrate-consuming 

 apparatus represented in the flowering and fruiting mechanisms. 

 When the carbohydrates are exhausted, then the photogenic ap- 

 paratus becomes more active again. This brief mention suggests 

 that inhibitions and stimulations to growth of the photogenic 

 apparatus affect the growth and development of the fruiting 

 apparatus, but the influences operate in inverse ratios. Physical 

 and chemical laws are undoubtedly at the bottom of the proc- 

 esses, just as in animal development. Certain processes favor 

 inhibition of growth and development, while others favor 

 stimulation of growth and development. 



For example, activity of muscle is stimulated or inhibited by 

 activity in nerve ; or activity in nerve may be augmented or re- 

 tarded by activity in gland. The principle is the same whether 

 activity is measured by functional energy manifestation or by 

 mass change expressed in growth. It is far more important to 

 recognize that the conditioning factors in a complex plant, or 

 in an animal like man, are legion, and that they are balanced in 

 algebraic sums of positive and negative influences. Many of 

 these factors are known, but apparently many more are un- 

 known at the present time. These are some of the content of the 

 phrase "the growth impulse." 



If one studies the phenomena of growth from the standpoint 

 of the size and mass of the individual animal and the time in- 



