[sifton] bar of SANIO IN GYMNOSPERMS 85 



reached when lignified wall-substance is deposited even over the now thickened 

 rims of the primary pit-areas. Sanio's rims represent a system of rod-like or band- 

 like pectic thickenings of the middle lamella running transversely in the radial walls 

 and linked here and there by slightly curved, longitudinal, band-like, similar thicken- 

 ings (representing the lateral margins of primary pit-areas). 



Bailey (1) in 1919 published the results of researches undertaken 

 with a view to ascertaining the origin of the rims. He suggests the 

 identity of the bars of Sanio seen in Araucaria with the normal bands 

 of primary wall situated between the scalariform primordial pits in 

 ferns, and believes that rims and bars in other genera of the conifers 

 and in other groups as well are of the same character. His hypothesis 

 is based on the premise that when a scalariform gives place to a row 

 of shorter bordered pits the primordial scalariform is not similarly 

 cut up. He observes further that in some cases "the elongated 

 bordered pits become replaced by vertical rows of smaller pits which 

 are staggered so that the pits in one row alternate with those of the 

 next series. These pits are usually superimposed over nearly the 

 whole surface of the primary pit areas, and the thicker portions of the 

 middle lamella tend to anastomose or form a reticulum." This, he 

 suggests, accounts for a normal lack of typical bars of Sanio in forms 

 where alternate pitting is strongly developed and makes it difficult 

 to explain how forms with these structures well developed can be 

 descended from ancestors with alternate pitting. In a number of 

 Angiosperms he finds evidence to corroborate his theory, the pits 

 being arranged in long, horizontal rows with bars between, so that 

 the structure of the primary wall is suggestive of that found in lower 

 forms with scalariform bordered pits. 



In 1915 the writer (10) discovered bars similar to those previously 

 noted in Araucaria, in the petiole of Cycas, and later (11)^ recorded 

 such structures in the secondary wood of Dioon spinidosum. Their 

 presence in forms, which are recognized as among the most primitive 

 of living seed plants and which cannot be held to have descended fi'om 

 Abietineae, suggested the correctness of Thomson's idea that this 

 type might be the primitive one from which the Abietineous form 

 originated. It was, therefore, considered worth while to investigate 

 the types of rims or bars throughout the Gymnosperms in the light 

 of this additional knowledge of their occurrence. For convenience 

 of reference certain of the photographs formerly published have been 

 repeated in this paper. 



As indicated by the title, the primordial pit of Sanio will be 

 given a considerable amount of attention since rims of Sanio have 

 not been found apart from these thin places in. the primary wall. 



