XLIV] THYLLOXYLON 243 



often lined by a layer of small cells (fig. 731). In the presence of 

 these large medvUaiy lajsAnomaloxylon lesemhlesThylloxylon, but 

 in the latter genus the rays are smaller and more uniform in size. 

 Gothan discusses the nature of these medullary rays and inclines 

 to the view that they agree more closely with abnormal or traumatic 

 formations in certain Conifers than with any normal structures. 

 There are no pits on the horizontal or tangential walls of the ray 

 cells and there are 2 — 3 simple circular pits in the field. 



The general impression gained from an examination of Gothan's 

 photographs is that no true canals occur, and that the peculiar 

 medullary rays owe their form to partial decay of abnormal 

 patches of parenchyma possibly produced as the result of wounding. 



Though on the whole nearer in structure to the Taxodineae^ 

 than to any other family Anomaloxylon is a type which cannot 

 be assigned to a definite position. 



XIV. THYLLOXYLON. Gothan. 



Thylloxylon irregulare Gothan. The generic name Thylloxylon 

 was given to a single species of Upper Jurassic age from Spitz- 

 bergen^ on account of the occurrence of tiillen-like parenchyma 

 in horizontal canal-like spaces in some of the larger medullary 

 rays. The wood is characterised by separate bordered pits in the 

 summer tracheids and 1 — 2 rows of alternate contiguous, Arau- 

 carioid, pits on the spring elements; xylem-parenchyma occurs 

 only at the end of the year's growth. The medullary cells have 

 Abietineous pitting and there are 2 — 3 small circular, apparently 

 simple, pits in the field, or occasionally only one in the region 

 of the late wood. The rays are uniseriate or 2 — 3 cells broad and 

 some medullary rays closely resemble those of certain Abietineae 

 possessing horizontal resin-canals ; but in Thylloxylon there are no 

 true canals. The central parenchyma of some of the broad rays 

 is replaced by a canal-like space and these spaces are often filled 

 with spherical tiillen-like tissue, a feature shared with Anomalo- 

 xylon, but in that genus there is no Abietineous pitting on the 

 medullary-ray cells. There are no vertical resin-canals. 



1 That is Athrotaxis, Oryptomeria, Sequoia, Taxodium, and Sciadopitys, genera 

 which are now (see page 126) assigned to different families. 

 ■ 2 Gothan (10) p. 34, PI. vi. figs. 2—8. 



16—2 



