FIBRO VASCULAR TISSUES: PARENCHYMA 49 



It will be clear from the account of the appearance of paren- 

 chymatous cells given in the preceding paragraphs that their origin 

 in the primary tissues of the wood is revealed only in the case of that 

 very ancient group of cryptogamous trees, the lepidodendrids of 

 the Paleozoic age, where they are very clearly derived from the 

 septation of tracheids. Further, in the case of the secondary wood, 

 parenchymatous elements properly so called did not make their 

 appearance before the Mesozoic period and are as distinctly corre- 

 lated in their origin with the phenomenon of annual rings pre- 

 sented by plants of the Mesozoic and later geologic time as are 

 tangential pits in the tracheids. As has been made clear in an 

 earlier chapter, the secondary wood of Paleozoic gymnosperms was 

 characterized, not only by the absence of annual rings, but equally 

 clearly by the default of pits on the tangential walls of the tracheids 

 and the entire absence of parenchymatous elements in the wood. 

 The arrival of the phenomenon of annual rings can be most satis- 

 factorily explained in the case of the known facts by the hypothesis 

 of the gradual refrigeration of the surface of our earth, with the 

 consequent appearance of a winter period of rest in vegetative 

 activity. The phenomenon of annual growth soon brought into 

 prominence the two striking features of terminal tangential pitting 

 and terminal storage parenchyma. Both features of organization 

 were later extended with greater or less completeness to the whole 

 of the annual ring. It is further quite clear that parenchymatous 

 storage elements came into being in the secondary wood by the 

 septation of tracheids, precisely as has been shown to be the case 

 in the similar elements of the primary wood (apparently only clearly 

 indicated in the case of the lepidodendroid cryptogams of the 

 Paleozoic age). 



In the higher gymnosperms, including the greater number of 

 conifers as well as the Gnetales, the distribution of the longi- 

 tudinal storage elements of the secondary wood is typically and 

 primitively diffuse; that is, such cells are not confined to the end of 

 the recurring zones of growth, but are scattered throughout the 

 annual ring. It is apparently not necessary to figure the situation 

 for the Gnetales, since it corresponds so closely with that found in 



