90 ANATOMY OF THE GYMNOSPERMS 



interchangeable relations are peculiar to the highest types of the 

 genus Finns. But we may ask, What is the function of these 

 structures which make their appearance only in the higher Coni- 

 ferae ? What is the proper significance of their appearance there, 

 and do any other plants offer parallel examples ? 



In the so-called medullary rays of Lepidodendron selaginoides 

 (81, 141) there are numerous reticulated or spiral elements which 

 are undoubtedly of the nature of tracheids, and they may beheld 

 to represent the ancestral form of the ray tracheids in the Coniferae, 

 toward which they bear the same relation that exists between 

 the spiral protoxylem element and the characteristic wood tracheid 

 with bordered pits. From this it is apparent that the ray tracheid 

 of Pinus or Tsuga represents a primitive structure which reappears 

 in response to conditions of growth and structural alterations of 

 such a nature as to demand the interposition of more simple, 

 because more primitive, elements for the proper performance of 

 necessary functional activities. These activities, in the case of 

 Lepidodendron, are probably expressed in the radial distribution 

 of water (81, 141), and we are no doubt correct in assuming 

 similar activities to be carried on in the higher Coniferae. In all 

 those species which present the primitive structure of the thin- 

 walled ray cells, both fossil and recent, there are no tracheids to 

 be found. As a tendency to thickening of the wall arises, there 

 is also developed a sporadic tendency to the development of ray 

 tracheids, as in Thuya and Cupressus. It is also a noteworthy 

 fact that simultaneously with a general thickening of all the cell 

 walls throughout the ray, as in the genus Tsuga, ray tracheids 

 become a constant and prominent structural feature. This rela- 

 tion exists in Pseudotsuga, Larix, Picea, and Pinus, and it is a 

 remarkable fact that as the type of organization advances, and 

 the structural modifications of the wall become more profound, 

 the tracheids gain steadily in numbers and importance until they 

 finally replace the parenchyma cells more or less completely. 

 Such facts serve to direct attention to the idea that by such pro- 

 gressive alteration the ray cells gradually lose their normal 

 functional powers with respect to the radial distribution of water, 



