342 THE ANATOMY OF WOODY PLANTS 



Normal and traumatic resin canals and traumatic ray-tracheids 

 are abietineous structures occurring in certain Taxodineae and 

 Cupressineae, facts which, in accordance with the general prin- 

 ciples of comparative anatomy already elucidated, may be regarded 

 as indicating the abietineous origin of the two coniferous subtribes 

 in which they occur. This conclusion is now somewhat generally 

 accepted by those whose anatomical knowledge of the conifers 

 makes their opinion of weight. 



Before we take leave of the two subtribes considered in the 

 foregoing paragraphs it will be well to direct attention to the 

 distribution and origin of wood parenchyma in woods of this 

 type. The secondary xylem is characterized by the presence 

 of usually abundant wood parenchyma, not confined to the end 

 of the annual rings, but scattered throughout. The parenchyma- 

 tous elements secrete a generally highly antiseptic essential oil. 

 As a result of the presence of essential oils, and sometimes also 

 by the infiltration of the tracheary walls with tannin, woods of 

 taxodineous and cupressineous origin are frequently resistant to 

 decay. The oil-secreting cells in the subtribes under discussion 

 do not under normal conditions betray their derivation from 

 tracheids except by the fact that they are grouped in series which 

 have the fusiform shape of tracheary elements. In injured woods 

 it is often possible to observe transitions between septate tracheids 

 and rows of parenchymatous elements. It may accordingly be 

 assumed, independently of the evidence of abietineous affinities 

 supplied in the previous paragraphs, that the storage elements 

 in the woods of the Cupressineae and Taxodineae are of tracheary 

 origin. Another feature must be considered in this connection. 

 In the minds of those who regard the Coniferales as an ascending 

 series and not one of reduction, the resin canals which characterize 

 the wood structure of the older Abietineae owe their origin to 

 the clustering of the resin cells of the cupressineous or taxodineous 

 type. This view of the origin of secretory canals in the coniferous 

 series has been particularly emphasized by Penhallow. It meets 

 with numerous difficulties, the chief of which is that the cells 

 which surround the resin canals are not resin cells. The latter 

 possess dark-brown contents and produce their secretion in an 



