THE ANATOMY OF WOODY PLANTS 



subtribes, such as the Cupressineae, Taxodineae, and Taxineae, 

 even the intercommunicating simple pits of the horizontal and 

 vertical walls of the ray cells are clearly and often conspicuously 

 absent. In such cases, as is to be expected, the wall of the ray 

 elements is in general thinner and often curved. 



An interesting condition of organization of the radial paren- 

 chyma is presented by 

 Chamaecy parts nootkaten- 

 sis. Here, as is shown in 

 Fig. 52, the rays are fre- 

 quently marked by the 

 presence of tracheary ele- 

 ments on one or both 

 margins. The illustration 

 represents the radial 

 aspect of the transition 

 from summer to spring 

 wood, and the features of 

 structure are in general 

 such as one would expect 

 to find under the circum- 

 stances, except as regards 

 ray organization. The 

 normal presence of margi- 

 nal tracheids in the species figured gains a special evolutionary sig- 

 nificance from the fact that similar conditions are found in other 

 representatives of the Taxodineae, Cupressineae, and the genus 

 Abies among the Abietineae as a consequence of injury. For 

 reasons which will be fully discussed in a subsequent chapter it 

 seems quite clear that the structures which are found to appear as 

 a result of injury in vascular plants with secondary growth are often 

 of the nature of reversions to an ancestral condition. Fig. 53 illus- 

 trates in radial view the wood formed after injury in a root of the 

 Big Tree (Sequoia gigantea) . Parts of two annual rings are included, 

 and the ray clearly shows features of structure which are abnormal 

 for the Taxodineae. To the left below and in the spring wood can 

 be seen several cells included in the substance of the ray which are 



FIG. 52. Ray of the Nootka cypress, 

 scription in the text. 



De- 



