1920] WRIGHT— PIT-CLOSIXG MEMBRANE 245 



A peculiar condition is occasionally met with in the stem. The 

 tracheids are more or less discolored when cut, and stain in a 

 peculiar manner. With Pianeze's stain the membrane becomes 

 yellow. It is usually uniform in thickness, but swollen, occasion- 

 ally almost entirely filling the pit (fig. 13). 



In the petiole of Botrychium, as in that of Hclminthostachys, a 

 uniform membrane prevails. With the exception of the petiole, 

 therefore, and peculiar unnatural spots in the stem, the typical 

 pit membrane in Botrychium has a torus. 



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Thus the only torus I have found among the cryptogams 

 occurs in forms whose pits are broad-bordered and circular or 

 oval in shape. Strasburger (8) makes the statement that a 

 torus occurs in Pteris aquilina, but he neither enlarges on the 

 statement nor illustrates it. DeBary (3) describes and pictures 

 for Pteris an almost imperceptible one-sided swelling of the mem- 

 brane, lying to one side of the pit and acting, he states, as a lid 

 to the pit pore. I have searched in vain for such a torus. Fre- 

 quently the membrane may have a "kink" toward the pit pore 

 simulating the appearance of a torus, but both its edges follow 

 the curve to an equal extent, thus precluding the possibility of a 

 thickening at that point. In Pteris the membrane in the pits 

 between tracheid and tracheid invariably remains uniform in 

 thickness. As has been shown in Botrychium, a plano-convex 

 torus such as DeBary describes may occur in the pits of a tracheid 

 where it touches a ray cell. In Pteris, however, the membrane 

 even in this region remains consistently uniform. Equisetum, 

 Psilotum, and Isoetes, forms with narrow-bordered pits of the 

 scalariform type, and a number of ferns (including Ophioglossum), 

 with the same type of- pitting, all show a definitely uniform mem- 

 brane. In Helminthostachys and Botrychium, whose pits are 

 circular, broad-bordered, and round-pored, there is developed a 

 definite torus. Although this suggests a possible relation of the 

 torus to the form of the pit, the question of its relationship, whether 

 structural, ecological, or phylogenetic, is one on which it is hoped 

 more light may be thrown after a study of the nature and occur- 

 rence of the torus in the other groups of the plant kingdom. It is 

 interesting to note, however, that the form of the torus in Botrychium 



