GROWTH OF PLANTS 123 



formations of the equation, Robertson deduces from this formula 

 another, which is more convenient for calculation: 



log — -^ = kiti - to), 



in which (^i — ^o) represents the time necessary for the growing 

 organ to reach one-half its final size. For many calculations, it 

 is convenient to take relative values instead of absolute ones, 

 and to assume the final value of A to be 100. 



Besides Blackman's and Robertson's formulas, others have 

 been proposed. But all of them fail to express adequately the 

 progress of growth, since, in addition to internal conditions, the 

 increments per unit time depend also on environmental factors, 

 such as temperature, light, and supply of nutrient substances. 

 Consequently, these formulas represent the process of growth 

 only approximately, Robertson's formula having the advantage 

 that it attempts to cover the whole period of growth. 



The causes that retard and check growth are not fully under- 

 stood at present. Here one must consider the problem of the 

 effect of certain environmental factors, specific internal catalysts, 

 and the general senescence of an organism, which is still unsolved. 



29. Growth Hormones, Auxin, Its Action and Chemical Com- 

 position. — The growth of cells in the zone of elongation depends 

 most intimately upon the activity of the zone situated above it. 

 If the top of the growing stem is cut, the elongation of the zone 

 situated below it will be retarded and accelerates again only in 

 case processes of regeneration occur on the cut end. Such an 

 inhibition in growth can be clearly observed on peduncles after 

 removal of the flower bud, and on the coleoptile of cereals 

 protruding from the soil a few centimeters. 



The coleoptile represents the first leaf of grass seedhngs. It 

 is colorless and possesses the shape of a hollow cylinder passing 

 at the tip end into a complete cone. It is similar to a finger of a 

 glove and serves for boring through the soil that covers the seed 

 and protecting the tip of the first green leaf that grows up through 

 its cavity. The growth of the coleoptile is mostly at its base. 

 It is very rapid at the beginning, being greater than the growth 

 of the first leaf. Later on, its growth slows down; and then the 

 first leaf overtakes it, ruptures the tip, and appears outside. 

 After this, the growth of the coleoptile ceases completely. 



