234 THE EVOLUTION OF PLANTS 



ought to be placed on engineering principles. The 

 plan worked very well as long as there was not 

 too much increase in thickness of the wood and 

 bast inside. In some Sigillarias and Lepidoden- 

 drons, however, the growth in thickness was con- 

 siderable, so as eventually to burst the outer 

 framework, and here we find that the supporting 

 tissues were constantly renewed from the second- 

 ary cortex as growth went on. Thus at all stages 

 the necessary mechanical strength was provided 

 just where it was most wanted, on the outside of 

 the trunk. 



The subject of the arrangement of mechanical 

 tissue in ancient plants is an interesting one, 

 because strains due to gravity have been about 

 the same in all ages, so that we can easily com- 

 pare the corresponding adaptations in the oldest 

 cases known with those in plants now living. 



In all respects, wherever we have the means 

 of forming an opinion, the organs of the oldest 

 fossil plants prove to have been just as well 

 fitted to then- work as in plants of our own times. 

 The leaves of Sigillaria and Lepidodendron had 

 two deep grooves on the under side, in which the 

 stomata were sheltered; these were further pro- 

 tected by hairs partly closing the grooves. Quite 

 similar devices are common in leaves of living 

 plants which are exposed to drought, the advan- 

 tage gamed being to check the too rapid loss of 



