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THE AMERICAN NATURALIST. [Vol. XXXVIIL 



Their strong tendency to obliteration in that genus is found to 

 coincide with the more marked development of ray tracheids. 

 which undoubtedly assume more completely the original func- 

 tions of the parenchyma cells, these latter in consequence, 

 suffering constant structural reduction, as in the hard pines. 

 In the distribution of the pits an important feature appears in 



the numerical variation in different parts of the ray. For diag- 

 nostic purposes it is necessary to have reference to the number of 

 pits, not upon the entire surface of an individual cell, but^within 

 the limits of a spring or summer tracheid as the case may be. 

 They are invariably most numerous in the region of the earliest 

 spring tracheids, usually diminishing toward the summer wood 

 where the change may sometimes take place abruptly, and*in 

 which they are most commonly reduced to one with occasional 

 obliteration in the most highly modified tracheids last formed. 

 A similar law of distribution is applicable within the vertical 

 hmits of the ray. When these structures are several cells in 

 height, the number of pits is typical, and, within certain narrow 

 limits, constant for all except the marginal cells. Thus if the 

 normal number is 1-2 for the central cells, it may sometimes 

 rise to 4, 6 or 8 in the marginal cells only, and such exceptions 

 must be noted in diagnosis. When the ray is only one cell in 

 height, the number of pits agrees with that for the marginal 

 cells. Such numerical variations possess but little value for 

 generic purposes, but as a specific character they may be held 

 to constitute the principal differential feature in the last analysis. 

 These relations are expressed typically in the genus Sequoia, 

 the two species of which may be definitely differentiated. S. 

 gigantea is characterized by oval and commonly narrowly 



