64 THE STEM 



I spent a day in making an estimate of its age, clearing away the 

 charred surface with an ax, and carefully counting the annual rings 

 with the aid of a pocket lens. The wood rings in the section I laid 

 bare were so involved and contorted in some places that I was not able 

 to determine its age exactly, but I counted over four thousand rings, 

 which showed that this tree was in its prime, swaying in the Sierra 

 winds when Christ \valked the earth." 1 



102- Types of adaptation. Plants are machines fitted to do work 

 under certain conditions. The work done by the plant is to take cer- 

 tain materials into itself, move them about, break them up chemically, 

 recombine them into new compounds, and build up its body, adding 

 to old parts and organizing new parts. Certain new parts finally 

 become new individuals. Growth and reproduction, and the moving 

 of materials for these purposes, are the work of the plant machine. 



The conditions under which the work is done are dependent upon 

 the nature of surrounding materials and the nature of certain forces 

 affecting the plant. Of materials, there are soil, water, and air ; of 

 forces, chiefly heat and light. Each of these conditioning factors 

 varies from place to place. The composition of the soil, the amount 

 and purity of the water, even the composition and density of the atmos- 

 phere, change as we go from one part of the earth's surface to another. 

 So, also, light is intense or feeble, and temperature high or low. 



Every new condition requires a new adjustment of the running 

 parts of the machine. It is peculiar to the machines which we call 

 plants and animals that they have the power of becoming adjusted to 

 new or changed conditions. Even in the individual plant there is 

 often seen a certain degree of the capacity for accommodation. When 

 we regard generations rather than individuals, this capacity becomes 

 still further apparent. Finally, when we look at the whole history of 

 plants we see that the plasticity of the plant machine is in the long 

 run perfect (within certain limits). Thus, plants become accustomed 

 to extremes of temperature. Arctic plants remain frozen for months 

 without harm. At a temperature very near the freezing point, arctic 

 and mountain plants are often active. On the other hand, tropical 

 plants resist heat. In the Punjab (India), air temperatures of 120 

 Fahr. are not uncommon. Schimper states that in a hot spring of 

 Venezuela certain low Algae thrive at above 176 Fahr. The vegeta- 

 ble machine, then, has the power of adapting itself in the course of 

 time to any kind of heat condition within the absolute death limits. 

 And heat is taken merely for illustration. Adaptation to light and 

 shade, or to variations of any other of the external factors of plant 

 existence, might have been given. 



Next, it is to be noted that plants of very different kinds often be- 

 come adapted to like conditions by taking on much the same structural 

 1 "The Mountains of California," by John Muir, p. 181. 



