154 MOKPHOLOGY OF SPERMATOPHYTES 



life are more favorable now than at any previous time in the 

 history of plants. 



The anatomy of the stem shows characters in common with 

 Pteridophytes, others suggestive of Dicotyledons, and still others 

 peculiar to the group. In all Gymnosperms the primary vascu- 

 lar bundles are open collateral, contain true tracheae, and are 

 organized into a hollow cylinder inclosing a more or less exten- 

 sive pith region. At this point the resemblance to the dicoty- 

 ledonous stem ceases. In many Cycads the primary cam- 

 bium is very short-lived, a succession of secondary cortical cam- 

 bium cylinders developing successive sets of cauline bundles. 

 This development of successive cortical cylinders of vascular 

 strands seems to represent a very ancient type of stem. It is 

 displayed by Gycas, Encephalartos, Macrozamia, and Bowenia 

 among the Cycads, belonged to the Bennettitales, and is appar- 

 ently shared by Gnetum and Tumboa. Furthermore, these sec- 

 ondary vascular bundles are of the concentric type in Gycas, 

 and give evidence in Macrozamia and Bowenia of a transition 

 from the concentric to the collateral type. The concentric type 

 of bundle is also displayed in the peduncles and leaves of other 

 genera, as Stangeria, Zamia, and G eratozamia, as well as in 

 BennettitaleSf It is significant, also, that this type of stem 

 characterized the so-called Cycadofilices. 



The gradual abandonment of the cortical cambiums, and the 

 conversion of concentric into collateral bundles, seems to have 

 taken place in the Cycads, as evidenced by Zamia, Dioon, and 

 Siangeria. In Cordaitales, Coniferales, and Ephedra, the pri- 

 mary cambium develops all of the secondary vascular tissue, and 

 the collateral type of bundle is fixed. It is interesting to note 

 that Cordaitales, the most ancient distinct group of Gymno- 

 sperms, are included in this last list, and it indicates an improb- 

 ability that there is any definite and single type of Gymnosperm 

 stem that can be regarded as the most ancient. 



The development of tracheids with bordered pits has long 

 been regarded as a distinctly Gymnosperm character, but they 

 seem to be wanting in Tumboa, and are said to be present in 

 certain fossil Pteridophytes. In any event, it is a type of vessel 

 displayed almost exclusively by Gymnosperms, and almost with- 

 out exception among them. A transition from tracheids with 

 bordered pits to true tracheary vessels seems to be evident in 



