1920] SIFTON—CYCADS 



429 



by 



elimination 



is illustrated in the tracheid to the right of fig. 11. The lighter 

 colored pits have typical bilateral borders, while the obscure ones 

 are unilateral, the corresponding pit on the overlying tracheid not 

 having been formed. 



I The arrangement of pits, opposite or alternate, deserves notice. 



J In fig. 6 two or three pits occur in a horizontal row. In some cases 



as many as four such pits have been found. In the more common 

 condition, not figured, the pits are regularly alternate, sometimes 

 round, and sometimes flattened by mutual contact. Just as the 

 scalariforms of one tracheid are horizontal while those of the next 

 are inclined, and either straight or curved and irregular (fig. 1), so 

 on one tracheid is found the opposite arrangement of pits, and 

 adjacent to it the alternate. There seems no reason for believing 

 that alternate pitting is formed by any disarrangement of rows of 

 round bordered pits. More specialized plants have one or other 

 of these types predominating; for example, the alternate and 

 flattened arrangement in Araucarineae, and the scattered, grouped, 

 and opposite in the Abietineae. The presence of all these condi- 

 tions in mature Cycad wood, as well as in the ancient fossil form 

 described by Gothan, modifies to a great extent their phylogenetic 

 significance in higher forms. It strengthens Bailey's statement 

 that the presence of opposite pitting as well as alternate in the cone 

 axes of Araucarians cannot properly be used as an argument for 

 their descent from the Abietineae, and neither is the same condi- 

 tion in primitive parts of pines an evidence of descent from an 

 Araucarian type. It would seem that if these facts have any 

 significance in phylogeny, they indicate that both pines and Arau- 

 carians are descended from lower forms which contained both 

 these arrangements. 



It may be stated in passing that both opposite and alternate 

 arrangements of pitting occur in the Cordaiteae, the alternate, 

 however, being greatly predominant. Instances of opposition 

 pitting in Cordaites may be seen in fig. 5, in the upper part of the 

 right hand pitted tracheid. There is an example also near the 

 lower end of the second tracheid to the left of it. Instances of 

 Gothan's grouped arrangement are also present, especially in the 



