GINKGOALES 



[CH. 



slightly flattened and alternate, but usually opposite (fig. 631, 

 E, F). The pores of the pits are often crossed. Eims of Sanio 

 form a well-marked feature on the tracheal walls, and JefErey^ 

 points out that they are clearly shown in the mature wood but 

 not in close proximity to the primary xylem or in the wood of the 

 reproductive shoots and leaves. True bars of Sanio frequently 

 occur on the tracheids^. The secondary phloem consists of dis- 

 continuous rows of fibres in addition to sieve-tubes and parenchyma. 

 Characteristic features are presented by the uniseriate medullary 

 rays : these are often 1 — 2 or 1 — 5 cells deep and do not appear 

 to exceed 11 cells in depth; they are comparatively large and in 

 tangential sections of the wood present an inflated appearance. 

 There are 2 — 7 elliptical pits in the field of the ray-cells. Xylem- 

 parenchyma though not abundant occurs here and there among 

 the tracheids ; the cells have thin walls and are larger than the 

 medullary-ray cells and characterised by the occurrence of stellate 

 calcium oxalate crystals^. 



B. Fossil. 



i. Petrified Wood referred to the Ginkgoales. 



The characters of the wood of Ginkgo hiloba summarised 

 above are in general agreement with those of many Conifers, 

 and such anatomical features as have been described by authors 

 as more or less distinctive of the genus do not afford very trust- 

 worthy guides to the identification of fossil wood. The compara- 

 tively large size and rounded contour of the medullary-ray cells, 

 as seen in tangential section, though worthy of note as character- 

 istic features, are hardly satisfactory criteria when applied to wood 

 that may have undergone partial decay and been exposed to 

 influences affecting the original form of the more dehcate tissues 

 before petrification. The untrustworthy evidence afforded by the 

 size of the medullary rays has been emphasised by Essner* who 

 states that the ray-cells of Ginkgo are larger than those in any 

 genus of Conifers. It has been claimed by Fehx^ that Ginkgo is 



1 Jeffrey (12) p. ,548. ^ Mijller (90) PI. xn^. 



' For anatomical details, see also Kleeberg (85) ; Essner (86) ; Strasburger (01) ; 

 Seward and Gowan (00) B. ; Penhallow (07); Sprecher (07); Burgerstein (08); 

 Tupper(ll). ^Essner (86). = Felix (94). 



