i92o] SIFTOX—CYCADS 



427 



type of scalariform. Below the center of the figure is a scalariform 

 reaching from side to side. Its border shows constrictions at 

 two points, evidently the beginning of a division into three bordered 

 pits. In the portions of the tracheid above and below, complete 

 divisions and other incomplete ones are in evidence. The name 

 u fusion pits/' which has been applied to similar appearances, is 

 evidently a misnomer in this case. They plainly represent phases 

 in the breaking up of the ancestral scalariform rather than the 

 union of two of the more specialized bordered pits. The small 

 pits on tracheids a and b of fig. 1 in all probability are formed 

 from the narrow bordered scalariforms in a similar manner. 



Figure 4 is a much lower magnification of a longitudinal radial 

 section of the fossil stem of Lyginodendron Old hafnium, acknowl- 

 edged to be one of the most ancient of the seed plants. This 

 form had attained in the secondary wood of its stem the condition 

 represented in Gothan's second type, the pits being practically 

 eliminated from the tangential walls (cf. Scott 8), but crowding 

 the radial walls from end to end of the tracheid. Wherever the 

 cell wall is present in the figure it is seen to be completely covered 

 with the type of pitting known as recticulate. A careful examina- 

 tion of the pits shows them to be of the same type as those in fig. i, 

 which had their origin in the narrow bordered scalariforms. 



The stem of Cordaites (tig. 5) has pits which, like those of the 

 Cycads, have probably originated from the cutting up of wide 

 bordered scalariforms, a condition quite in keeping with the 

 general higher type of wood structure exhibited in the Cordaiteac 

 forms. 



Further evidence of the origin of the bordered pit from tht 



'Calariform 



A type of fusion pit which recalls the condition in the narrow 

 bordered scalariforms of fig. 1 is shown in fig. 10, which is a radial 

 section of the stem of Dioon spinulosum. The three pits nearest the 

 top are of the short, slightly flattened type quite common in these 

 forms. The next three are more elongated. All show the charac- 

 teristic cross pores of adjacent elements. The seventh of the 

 series is a pit of the second fusion type. It appears like two pits, 

 each with a short pore, with a common long pore crossing both. 



