142 CONIFEEALES (BECEITT) [CH. 



are usually termed transfusion-tracheids^ is regarded by some 

 authors as homologous with the centripetal wood of Cycadean 

 leaves (Vol. iii. p. 31 ) and Bernard definitely adopts the term centri- 

 petal xylem. Jeffrey^ and, more recently, Takeda^ do not attach 

 a similar morphological significance to the short tracheids, and 

 they are probably justified in their sceptical attitude. The 

 transfusion tracheids are often reticulately pitted : in many leaves 

 they form conspicuous flanges on the sides of the vascular bundles 

 (fig. 694, B, C, t) or they may more or less encircle the vein. In 

 some leaves, e.g., Araucaria, it is noticeable that the amount of 

 transfusion tissue (fig. 694, C, t) increases as the vein is traced 

 towards the leaf-apex until the long and narrow elements may be 

 entirely replaced by a group of short transfusion-tracheids. 

 Another type of accessory tracheid is occasionally met with, 

 namely elongated tracheids traversing the mesophyll between the 

 veins and the edge of the lamina. This is seen in the long and 

 narrow leaves of some Podocarps*. 



The anatomy of cone-scales is too wide a subject for adequate 

 treatment in this sketch. The general rule is that in each scale 

 there are two sets of vascular strands, a lower set of normally 

 orientated bundles and an upper inversely orientated series. In 

 some cone-scales, e.g., Araucaria, concentric vascular strands are 

 a prominent feature. The taxonomic significance of the ana- 

 tomical characters of cone-scales has been discussed by Radais 

 and more recently by Miss Aase^, to whose accounts the student 

 is referred. 



It has been shown that characters which it has been customary 

 to associate with a definite type of wood may occur sporadically 

 in several other Conifers ; but this does not invalidate conclusions 

 based on the prevalent occurrence of such features in a given 

 specimen. It is untrue to say that contiguous and alternate pits 

 are the monopoly of the Araucaiineae and it is incorrect to assert 

 that in Araucarian wood the pits are never separate. Similarly 



1 So named by von Mohl; see Wordsell (97); Bernard (04) B. ; Carter (11). 

 - Jeffrey (OS^). 



3 Takeda(13); see also Thomson (13). 



* For further details with regard to leaf-anatomy, see Thomas, F. (66) ; Ber 

 trand, C. E. (74); Mahlert (8.5); Striibing (88) ; Daguillon (90). 

 5 Radais (94); Sinnot (13); Aase(15). 



