252 0. A. Derby — Stem Structure of Tietea Singularis. 



possessed six such rows, while if the outline was oval the num- 

 ber would be eight. An exceptionally fine and complete 

 Psaronius stem in the collection of the Brazilian Geological 

 Service, which will shortly be described by Prof. Pelourde 

 of the Paris Museum, shows that the oval outline indicated by 

 the fragments of several other species of Psaronius is an origi- 

 nal feature, and not, as has generally been assumed, one due 

 to the distortion of the trunk after the death of the plant. In 

 this latter stem the number of rows is sixteen, and this seems 

 to indicate that in the polystichous Psaronides (the ill-defined 

 continuation of the di- and tetrastichous groups) multiples of 

 four should be expected rather than haphazard numbers. On 

 this hypothesis an oval eight-rowed stem rather than a circular 

 six-rowed one should be postulated for Tietea singularis, and 

 in accordance with this view the accompanying hypothetical 

 restoration (fig. 1) has been made. As thus restored the stem 

 has about the same dimensions as the sixteen-rowed Psaronius 

 above mentioned with a less departure from the circular form 

 normal to most tree trunks. 



In fig. 1 an oval of the proper dimensions, deduced from the 

 peripheral curve of the slices with exclusion of the root sheath, 

 has been traced to represent the top of the missing lower part 

 of the trunk from which the original specimen had been 

 broken. Within this oval the lowest cross section at hand (14) 

 has been fitted where it best conformed to the curve of the 

 oval, and three of the most significant of the overlying sections 

 have been projected down on to the same plane with their pe- 

 ripheries adjusted to the remaining portions of the curve. This 

 gives schematically the general aspect of a complete cross sec- 

 tion of the stem at plane 14, on which, of course, the vascular 

 groups would take the numbers 1 to 8, and, except in the 

 shaded portion, would present details different from those 

 here shown. Of the sections utilized 2 and 10 are represented 

 photographically in Count Solms-Laubach's paper.* 



Comparing this cross section with those of the several known 

 representatives of the polystichous group of Psaronius, a funda- 

 mental agreement is found in the threefold character of the 

 vascular system, accompanied by a fundamental difference due 

 to the composite character of both the peripheral members of 

 that system. It may be suspected that this difference will 

 eventually prove to be of more than generic importance. 



In fig. 2 corresponding portions of eight of the cross sec- 

 tions are plotted in sequence with a forward inclination of 15° 



* The comparative numeration used in the present paper and in that above 

 cited is as follows : I = V, 2 = IV6 (fig. 5), 3 = IVa, 4 = 1116, 5 = Ilia (fig. 

 4), 6 = 116 (fig. 3), 7 = Ha (fig. 2, which was reverted by the photoengraver), 

 8=1 (fig. 1), 9= -I, 10 = IIa (fig. 6), 11= -116, 12 = -Ilia, 13=-III6, 14 

 = -IVa. 



