XLin] ANATOMY 129 



well defined rings of growth should be noted and attention paid 

 to the breadth of the late summer ('autumn') wood: Goeppert^ 

 considered the breadth of annual rings a character of importance, 

 but Kraus^ and others have shown that this is of little significance. 

 In the Cupressineae (in the more restricted sense) it is probably 

 true that the rings are generally though not invariably narrower 

 than in Abietineae : in roots the later wood is smaller in amount 

 and there is a more sudden transition to the spring-wood than in 

 stems*. Though as a rule there is a considerable difference in 

 the thickness between the walls of the spring and summer tracheids, 

 in Podocarpus Nagi* the difference is slight. In some species 

 of Araucaria the rings are absent or feebly marked, a fact noticed 

 long ago by Nicol^, and in other Conifers, e.g., Widdringtonia 

 juniperoides and Tetraclinis^ there may be no definite rings; in 

 Libocedrus macrolepis'' there is but little difference in the thickness 

 of the spring and summer tracheids. It is, however, impossible 

 to say to what extent this is an inherent tendency and how far it 

 reflects the influence of external conditions: it may be that the 

 frequent absence of rings in Araucarian wood is explicable on the 

 hypothesis that this family is the oldest and most closely related 

 to Palaeozoic types, which are almost invariably characterised by 

 an absence of rings : the habit of forming well defined spring- and 

 late summer- wood may have been acquired at a later stage ^. The 

 interest of annual rings is rather biological than taxonomic and it 

 is chiefly in connexion with fossil plants as tests of climate that 

 attention has been directed to this feature*. 



The genus Taxus is peculiar in having no resin-ducts in the 

 cortex or stele of stem and root or in the leaves. In some genera 

 resin-canals are a constant feature in the secondary wood, e.g., 

 Pinus, Picea, ' Larix, Pseudotsuga ; while in other Abietineae 

 canals do not usually occur in the xylem. This distinction is, 

 however, by no means constant and, as Jeffrey^" has shown, the 



1 Goeppert (50). ' Kraus (64) p. 146. 



' Gothan (10) p. 11 ; Penhallow (07) p. 31 ; von Mohl (62). 

 * Fujloka (13). ^ Nicol (34) A. p. 139. 



6 Conwentz (90) A. p. 33. 



' Fujioka (13) p. 213. « Thomson (13) p. 33; Gothan (07) p. 25. 



9 Seward (92) B; Gothan (08^); Antevs(16); (17). 

 " Jeffrey (03) ; (05); etc.; Penhallow (07) pp. 123 ef se^- J Jones (13^). 



s.iv 9 



