^UH] ANATOMY 131 



have lost the power of reversion retained by Sequoia and Abies 

 are placed higher in the evolutionary series. The vestigial signi- 

 ficance of resin-canals is by no means generally admitted. Pen- 

 hallowi holds, and I beheve rightly, that they are not primitive 

 features ; their occurrence in the young shoots of certain species 

 and in .the peduncles of cones but not in the older wood may, as 

 Gothan^ suggests, be correlated with a greater need of protection. 

 Kirsch^ considers that the development of canals in young wood 

 and in peduncles may be connected with the relatively greater 

 abundance of food in those regions which, in his opinion, would 

 induce a greater production of parenchyma and secretory passages. I 

 Moreover, if the occurrence of canals in the axis of a female cone i 

 of Sequoia gigantea is attributed to the retention of an ancestral ! 

 character, why do not canals also occur in the axis of the micro- | 

 strobiK? The facts demonstrated by Jeffrey and his pupils are 

 of great interest, but considered by themselves they may equally 

 well be interpreted as favouring the greater specialisation and 

 more recent development of those genera in which the production 

 of resin-canals is a normal character. 



The structure of the epithelial cells is employed as a taxonomic 

 character though, as Conwentz* suggests, it is not a very satis- 

 factory criterion and in petrified tissues it is often difficult to 

 distinguish between true thick walls and walls thickened by 

 secondary deposits. In Pimis the walls of the cells lining the 

 canals are frequently thin^, but in some species thick ; Larix and 

 Picea have thick- walled epithelial cells. The occurrence of tyloses, — 

 the parenchymatous cells that invade the cavities of water-con- 

 ducting elements,^has generally been regarded as the monopoly of 

 Angiosperms : though unknown in recent Ferns they occur in some 

 extinct types. Chrysler^ has shown that tyloses are produced in 

 the tracheids of Pinus, apparently as a consequence of wounding. 

 Tyloses have also been found in some fossil coniferous woods. 



The arrangement of the bordered pits on the radial walls 



of the tracheids is the character to which most attention has 



.been given. In the Abietineae they form either single or double, 



1 PenhaUow (07) p. 150. 2 Qothan (07) p. 40. 



' Kirsch (11); Thomson (13) p. 38. See also Burlingame (1.5^). 



^ Conwentz (90) A, p. 45. * Groom and Rushton (13). 



8 Chrysler (08) B. p. 204. 



9—2 



