FIBROVASCULAR TISSUES: TRACHEIDS AND FIBERS 27 



Tangential pits are so obviously and constantly a criterion of 

 structure at the end of the annual rings in conifers that they may 

 be used for the purpose of distinguishing the yearly increments in 

 tropical species, in which, by reason of the slightly marked seasonal 

 conditions, the zonal woody bands are indistinctly indicated or even 

 apparently absent. This is notably the case, for example, in the 

 tropical or subtropical species of the genus Araucaria. 



The phenomenon of annual rings is very closely correlated with 

 the appearance of tangential pits, and the general phenomena in 

 this respect are worthy of much greater attention than has hitherto 

 been devoted to them. The incipient yearly zones in trunks of 

 trees of earlier ages are the clearest indication of the progressive 

 climatic refrigeration of the earth, and these become ever more 

 marked in later geological times. Secondly, it will be made clear in 

 the sequel that the modification of the annual ring in response to 

 inclement seasonal conditions has been on the whole the most 

 important factor in the evolutionary development of plants from 

 the earlier epochs to the present. A consideration of the organiza- 

 tion of the wood in a gymnosperm without annual rings that is, a 

 seed plant of Paleozoic time is of extreme interest in the present 

 connection. The tracheids of the secondary wood of the gymno- 

 sperms of this age, without exception so far as is at present known, 

 had the pitting confined to the radial walls; as a consequence water 

 could move, easily at any rate, only in a spiral and tangentially 

 through the tracheary elements. The truth of this situation may 

 readily be grasped by reference to Fig. 1 1, in chapter iii, representing 

 wood of the Paleozoic gymnosperm Cordaites. Details of the 

 structure of the wood in this genus can be gathered from the 

 inspection of Fig. 22 in the present chapter. It is clear that 

 the tracheids communicate with one another and with the cells of 

 the uniseriate rays by radial pits. Pits are conspicuously absent 

 on the tangential walls of all the fibrous elements of the wood. It 

 will be obvious from the facts put forward in this paragraph that 

 the distinction between spring and summer tracheids did not 

 exist in the case of Paleozoic woods; in other words, the modifica- 

 tions in structure and pitting which have become a fixed feature of 

 the organization of the summer tracheids of trees of the Mesozoic 



