BRIGGS: THE LIVING PLANT AS A PHYSICAL SYSTEM 



97 



our common plants varies as an exponential function of the time, 

 in so far at least as such changes in size are reflected by the 

 change in transpiration through a series of uniform days. In 

 other words the young plant confronted with the problem of 

 maturing its seed and completing its life cycle before the ad- 

 vent of frost, proceeds to develop its system at the maximum 

 possible rate consistent with the conditions, i.e., in accordance 

 with the compound interest law. When an adequate leaf system 

 is secured, the plant apparently turns its attention to the elabora- 

 tion of material for seed production and the rate of increase in the 

 size of the leaf system is modified. It is evident that any in- 

 hibiting factor, such as a limitation in the supply of water or 

 of nutrient solution also would bring about the same result. 



100 200 300 400 500 



1000 .1500 2000 



Age of ti-ees in years 



2500 



3000 



Fig. 3. Change in thickness of annual rings of Sequoia with the time. After 

 Huntington. 



Let us now turn from annual plants, which complete their 

 life cycle in a few months, to the other extreme, and examine the 

 rate of growth of the giants of the plant world, the Sequoias or 

 big trees of California. Huntington 4 has recently made extensive 

 measurements of the thickness of the annular rings of these trees 

 with a view to determining to what extent the variation in 

 thickness of these rings is correlated with fluctuations in the 

 weather conditions during the life of the tree. His measurements 

 showing the change in thickness of the ring as the tree grows 

 older, based upon measurements of trees of varying ages up to 



4 Huntington, E. The climatic factor as illustrated in arid America. Car- 

 negie Inst, of Washington, Pub. No. 192. 1914. 



