[pENHAi.Low] NOTES ON TERTIARY PLANTS Bl 



somewhat thick, especially ^o in two cases shown by an examination 

 of seven species of poplars and nine species of willows, though it is to 

 he noted that as between these two genera, the walls are much thicker, 

 and the structure as a whole is more dense in Salix than in Populus. 

 In the table already presented, comparison has been made with those 

 two species of poplar and willow which most nearly resemble the fossil, 

 while two indigenous species of Ehamnus ,are also brought into com- 

 parison. From this it will become apparent that so far as these cases 

 go, the structure of Rhamnus is composed of much smaller and thinner- 

 walled cells than in the Salicacege, and that the fossil distinctly ap- 

 proaches P. balsamifera. But we cannot draw final conclusions from 

 such data, since the willows and the poplars show that very considerable 

 variations in the size of the wood cells obtain as between one species 

 and another, and we have no evidence in support of the idea that an 

 Eocene or Miocene Rhamnus may not similarly have had relatively 

 large cells. 



In both Rhamnaceœ and the Salicacese, the growth rings show 

 s. very poorly defined summer wood which consists of 3-4 radially 

 flattened cells with somewhat thicker walls. In Rhamnus caroliniana 

 even this distinction is almost obliterated, but it persists in the Sali- 

 cacese as a whole, with considerable constancy. Both families possess 

 a strong point of resemblance in the numerous vessels which, in trans- 

 verse section, become abruptly smaller in the region of the summer 

 wood, are disposed in radial series, and also form groups in which the 

 cells fall into radial series of 1-7 or sometimes more. In the Salicaceœ 

 such radial extension is almost exclusively developed, but in the Rham- 

 nacege there is also a tangential extension of a more irregular character 

 whereby the series often broadens out into an irregular group. This 

 constitutes a somewhat definite dift'erential character, though one of 

 minor importance. In the radial and tangential sections the vessels 

 are of two kinds in Rhainnus, but only of one kind in the Salicaeese, 

 and this feature is one which acquires definite importance for differen- 

 tial purposes. Many of the vessels in Rhamnus and all the vessels 

 in the Salicacese show multiseriate, hexagonal pits which are rarely 

 oval, the pore of which is a transverse, narrow slit. In addition to these 

 the Rhamnaceœ also exhibit numerous and prominent sealarifonn ves- 

 sels with narrow and somewhat distant bars, a feature which is entirely 

 wanting in the Salioaceae. 



Longitudinal sections of the wood of Rhamnus show somewhat 

 prominent wood parenchyma in the region of the vessels. These ele- 

 ments are thin-walled and several times longer than broad in R. caro- 

 liniana, but rather thick-walled and 2-3 times longer than broad in 



