CONSIDERATION OF PLANT GROWTH 183 



measurements of a rapidly growing plant. Growth is not 

 related to photosynthesis, except for the food supply which 

 is normally in excess of the immediate needs. 



Inasmuch as all cells of an organism come from the origi- 

 nal fertilized egg and since all newly divided cells are similar, 

 it is obvious that cells must differentiate to form the many 

 kinds of tissues in an organism. This change from the origi- 

 nal kind of cell must be controlled by hormones or organizers 

 but it is not understood. Finally the cells take on their 

 matured characters, as the wood cells with thick lignified 

 walls or the thin cell walls of the pith or any one of the many 

 forms of cells in the leaves. 



The rate of growth is variable depending on the condi- 

 tions that influence growth. Normally a cell, an organ, or 

 complete organism, such as a plant or animal, begins to grow 

 slowly, but increases the rate for some time, following which 

 the rate remains high for a time and then declines. It is not 

 uniform but rhythmic. This is true of a shoot or of one side 

 of a shoot or of a pollen tube or a fruit. This rhythm is best 

 shown by time-lapse motion pictures. 



The rate of growth is influenced by temperature. The 

 lowest or minimum temperature for growth is about freezing. 

 As the temperature increases the growth rate increases to 

 about 90° F., beyond which it decreases for most plants. 

 Many plants that have been studied may grow rapidly for 

 a few hours at a high temperature after which the rate de- 

 creases, but if the temperature decreases for a time, growth 

 increases again with an increase in temperature. This may 

 be a night and day response. 



Many plants have a decreasing growth rate as the tem- 

 perature increases above 90°, and may stop growing at about 

 one hundred ten degrees. Plants can withstand higher tem- 

 peratures and will resume growth when conditions become 

 favorable. The transpiration of plants reduces their tem- 

 perature; therefore, if plants are killed by naturally high 



