344 



PLANT MORPHOLOGY 



largely iset'oudary in origin, show a collateral arrangement. They are 

 enclosed by a zone of "transfusion tissue." 



The stem of the Coniferales is an ectophloic siphonostele and exhibits 

 an advanced type of vascular anatomy (Fig. 294). It has a small pith, a 

 thick vascular cylinder, and a thin cortex. The organization of the con- 

 ducting tissues is collateral and endarch. . Only traces of mesarch struc- 

 ture remain, as in the cotyledons of certain genera. There is a great 



Fig. 294. Cross section of a six-year-old stem of Piniis monophyUa. showing small pith, 

 thick vascular cylinder, and thin cortex, XH. The phloem occupies a narrow zone out- 

 side the xylem, the latter showing radiating vascular rays and numerous resin ducts. 



development of secondary wood, which consists almost entirely of tra- 

 cheids. The tracheids have bordered pits, those of the Araucariaceae 

 being mostly in two or three rows, instead of in a single row as in the other 

 conifers. In Taxus the tracheids bear spiral thickenings in addition to 

 the bordered pits. The phloem of conifers consists chiefly of sieve tubes. 

 There are no companion cells. The leaf traces may be double, as in the 

 Abietaceae, or single, as in the Cupressaceae. The conifers are char- 

 acterized by the presence of resin canals, which are long intercellular cav- 

 ities lined with resin-secreting cells. They commonly occur in all parts 

 of the plant, being especially abundant in the leaves and in the cortex of 

 the stem. In Pinus and many other genera they may also be present in 

 the wood. In Taxus resin canals are lacking. 



The strobili are normally monosporangiate. The majority of conifers 

 are monoecious, but some are dioecious, e.g., Juniperus, Araucana, Podo- 

 carpus, the Taxaceae, and a few others. The arrangement of the sporo- 



