No. 448.] 



XORTH AMERICAN CONIFERALES. 25 I 



abruptly as in the modern Coniferae. The general tendency of 

 such evidence is to show that with a higher type of organization, 

 there is a corresponding diminution in the transition zone and 

 increased abruptness in the structural alterations. The logical 

 result of an extension of this process would be the reduction of 

 the bordered pit to the condition of a simple pit, and ultimately, 

 its complete obliteration. In ^ Coniferae the reduction of the 

 bordered pit to the condition of a simple pit sometimes occurs in 

 the case of medullary rays or even in the case of tracheids with 

 very thick walls, but it becomes most prominent in the angio- 

 sperms where it is a characteristic feature. Instances also occur 

 in some of the hard pines, in which the pit is completely obliter- 

 ated. This applies in particular to tracheids of the summer 

 wood, the walls of which have become unusually thickened. 



The relations to which attention has thus been directed some- 

 what in detail, have been expressed in more general terms by 

 DeBary (9, p. 321) in the statement that Outside the primi- 

 tive elements, wider trach^e follow. Their development takes 

 place successively, advancing from the inner edge of the bundle 

 outwards, and as a rule at a time when the elongation of the 

 entire part to which they belong is nearly at an end. The 

 thickenings on their walls therefore have a successively denser 

 arrangement : dense spiral and annular trachje, then reticulated 

 and pitted trach^e follow one another in succession from within 

 outwards, with gradual transitions, or with the omission of one or 

 the other immediate form." It is probably a justifiable inference 

 from the preceding facts that, the relation which exists between 

 the spiral tracheids of the protoxylem and the pitted tracheids 

 of the secondary xylem in the Coniferae, is, in general terms and 

 from the standpoint of development, the same as that exhibited 

 between the lower and higher types of vascular plants. 



Accepting the general principle which appears to be justified 

 by the foregoing facts, that the transition from spirals to bor- 

 dered pits is a feature in development which bears a direct rela- 

 tion to the evolution of higher types of organization, we may 

 utilize it for the purpose of determining the general phylogeny of 

 the Conifers so far as they may show a survival of such charac- 

 ters. Out of a total of 117 investigated species of indigenous 



