454 BOTANICAL GAZETTE [june 



and Pinus, are evidences of the Cordaitean ancestry of these genera. 

 He considers the opposite pitting and thickenings of the middle 

 lamella which occur in the cone axes of Araucarians to indicate that 

 the Araucarieae are descended from forms resembling Ginkgo and 

 Pinus. Furthermore, although admitting that "bars of Sanio" 

 are absent or "evanescent" in the young wood of seedlings and the 

 first annual rings of the stems and roots of Araucarieae, he interprets 

 the absence of approximation and consequent flattening of the 

 bordered pits in such tissue as evidence for deriving the Araucarians 

 from pinelike ancestors. 



The accuracy of these conclusions has been questioned by 

 Thomson (i6) and Sifton (14), who have figured and described 

 bandlike thickenings of the middle lamella in the tracheids of the 

 petiole of Cycas and the cone axes, seedlings, and first formed 

 secondary xylem of the stems and roots of Pinus and other Abieteae. 

 Thomson (16) interprets the rimlike thickenings and alternate 

 pitting that occur at times in the cone axes and first annual rings 

 of stems and roots of Pinus as indicating that the Abieteae are 

 descended from the Araucarieae. 



It is to be emphasized, in this connection, that in dealing with 

 other structural characters Jeffrey interprets the anatomy of 

 selected conservative regions or organs of the Abieteae and Arau- 

 carieae as indicating that the latter are descended from the former ; 

 whereas Thomson, by applying the same laws to similar material, 

 proves the reverse to be true. 



Such discrepancies as these suggest that there may be a con- 

 siderable element of danger in placing too much emphasis upon 

 "laws" of recapitulation, reversion, and retention in arguments 

 concerning the phylogeny of plants. Even the most ardent advo- 

 cates of these doctrines admit, in certain cases at least, that ceno- 

 genetic characters do occur in seedlings, roots, traumatic tissue, 

 cone axes, etc. So long as this is acknowledged to be so, it must be 

 extremely difficult, in the absence of reliable collateral evidence, 

 to determine with certainty whether a given structure in a given 

 region is palingenetic or cenogenetic. In other words, even if it 

 should be proven, by means of careful statistical and experimental 

 investigations, that certain organs or regions of plants are inherently 



