252 GYMNOSPERMS 



They make their greatest display in the Abietaceae, and, within 

 this family, in Pinus, Picea, Larix, and Psciidotsuga, where they 

 form an anastomosing system in the wood and cortex of both root 

 and shoot. In the rest of the family- Abies, Pscudolarix, Ccdrus, and 

 Tsuga — they are absent from the secondary wood of both root and 

 shoot. However, in these four genera, the canals are sometimes 

 found in the wood of the axis of the female cone, and they can be 

 produced in the secondary wood in response to wounding. 



In the Cupressaceae there are, normally, no resin canals in the 

 wood, except that in Sequoia giganlca canals appear in the wood of 

 the first year, and in the wood of the peduncle, axis, and scales of the 

 female cone, and also in the leaf traces of vigorous leaves. In 5c- 

 quoia sempervirens canals are absent from all these parts; but 

 Jeffrey-9° found that, as the result of injury, resin ducts appear 

 in the wood of both root and shoot. He concludes that 5. semper- 

 virens is a more recent type than S. gigantea and that the wounding 

 has made S. sempervirens revert — ^as far as resin ducts are concerned 

 — to the earlier condition of 5. gigantea. 



Resin cells — ^not ducts or canals — are a constant feature of the 

 wood of Taxodiaceae and Cupressaceae, especially in the later part 

 of the annual ring; but there are canals in the cortex. 



In the Araucariaceae there are resin canals in the medullary rays, 

 cortex, and leaves, and resin is laid down in tracheids bordering the 

 medullary rays. The abundant resin of Agathis australis is the basis 

 of a high grade of varnish. Some of the resin is obtained from living 

 trees, but most of it is mined on the sites of ancient forests where the 

 tree no longer exists. 



In the Podocarpaceae there are no resin canals in the wood or 

 cortex; but there are canals in the leaves, and parenchyma cells 

 containing resin are widely distributed. 



The Taxaceae present the least display of resin in the six families. 

 Cephalotaxus has resin canals in the pith and cortex, with resin-like 

 contents in some of the parenchyma cells of the wood. In Torreya 

 there is a large resin canal in the leaf, and parenchyma cells in the 

 wood contain resin. Taxiis has no resin canals at all; but resin cells 

 are reported for the roots of Taxus ciispidata and A itsirolaxits. 



From the foregoing it is evident that a series can be arranged from 



