GENERAL CHARACTERISTICS OF GROWTH 321 



cells but also in the organs which they compose. In animals 

 these three phases, including the grand period of growth, also 

 occur. 



Since the S-curve of growth is the same shape as that of an 

 autocatalytic process, i. e., of a process governed by an enzyme 

 produced as the reaction proceeds, Robertson proposed that 

 growth is governed by an enzyme developed within the cell at the 

 time of cell division. This enzyme diffuses (according to this 

 hypothesis) through the cell and stimulates more growth. But, as 

 the enzyme accumulates in excessive amounts, growth is checked. 

 Although plants do make their own enzymes, this hypothesis 

 is not only inadequate to explain the many variations in the 

 process produced by external factors but it is also unnecessary, 

 since the general S-shape of the growth curve can be more readily 

 explained on a nutritional basis. As the plant starts growth, the 

 reserves from the seed are slowly mobilized ; but as germination 

 advances the mobilization of reserves continues at a more rapid 

 pace, and the growth likewise continues at a rapid rate as the 

 leaves unfold and autotrophism starts. Later on in the life of 

 the plant, as sexual maturity is reached, the demands for foods of 

 the flowers, fruit, and seeds shut off the supply to the vegetative 

 organs, and growth decreases. Also, in woody plants, the greater 

 difficulties of transportation attendant upon the increased dis- 

 tance between the roots and leaves tend to cause a slowing down 

 in the growth rate. Thus, while enzymes play a part in all these 

 processes, there is no reason to attribute the various manifesta- 

 tions of growth to a single specific growth enzyme, as was pro- 

 posed by Robertson. 



The absolute rate of growth during the grand period varies in 

 different plants, but a few examples may furnish some conception 

 of the great rates which may be obtained. The stamens of wheat 

 (Triticum) at the time of blooming grow for a brief time at the 

 rate of 1.8 mm. a minute, which is about the speed of the minute 

 hand on a watch. If this rate continued for ten hours the stamens 

 would become over a meter long. The leaf sheath of the banana 

 grows at the rate of 1.1 mm. a minute, which is almost as fast, 

 and young bamboo shoots grow at times more than a foot a day. 

 When the century plant (Agave) blooms, as it does after 15-25 

 years, a tall flowering stalk about five meters high and 15 cm. in 

 diameter rises at the rate of about 15 cm. (6 inches) a day. These 



