APPENDIX A 611 



This phylum includes all those plants in which the sporophyte is the 

 dominant generation, with the gametophyte reduced, and in which the 

 sporophyte has a well-developed vascular system. It is the latter charac- 

 teristic which gives the name to the group. The old divisions Pteridophyta 

 and Spermatophyta are included. 



Subphylum I. Psilopsida (si lop' si da). This group includes the most 

 primitive of vascular plants, the extinct psilophytes and two surviving 

 tropical related genera. The oldest known true land plants are psilophytes 

 from the lower Devonian of Canada, but the group is better known from 

 the middle Devonian of Scotland, where a number of species of several 

 genera have been found. Two of these, *Rhynia and *Asteroxylon, are 

 shown in Fig. 26.9. The simplest were merely erect, sparsely branched 

 green stems 8 to 10 inches tall, that grew thickly clustered on swampy 

 ground. They lacked roots and leaves. The tips of some of the branches 

 were enlarged into hollow, oval structures in which were formed numerous 

 spores. The undiscovered gametophyte generation was probably, as in the 

 fern, a delicate and ephemeral structure. 



The stem of Rhynia contained a central strand of vascular tissue, surrounded 

 by cortical parenchyma, and a cutinized, stomata-bearing epidermis. In these 

 respects the psilophytes resembled ferns, but they were more primitive than 

 any fern, for there were no true roots; the stem continued under the ground as a 

 branching rhizome furnished with rhizoids, as in the bryophytes. Asteroxylon 

 was more advanced than Rhynia in that it possessed small scalelike leaves densely 

 covering the stem. The remaining subphyla of the vascular plants probably 

 developed from various psilophytes, and the lycopods in particular may well have 

 come from plants of Asteroxylon type. 



Subphylum II. Sphenopsida (sphen op' si da). 



The Horsetails and Their Allies. In this group the sporophytes have 

 roots, rhizomes, aerial stems, and leaves that are generally small, often 

 wedge-shaped, and arranged in whorls at the nodes of the stems and 

 branches. The stems are usually jointed. The spores are all alike and are 

 borne in sporangia on sporophylls that may be clustered or may form a 

 compact cone. 



The subphylum includes the single class Equisetinae, with three orders, of 

 which only the Equisetales have living representatives. These all belong to the 

 genus Equisetum (horsetails or scouring rushes, Fig. A. 13), of which there are 

 about 30 species. Most of them are less than 18 inches tall, though in Central 

 America and Cuba there is a giant form that grows in dense stands to a height of 

 •40 feet; its stem diameter, however, is never more than an inch. All the members 

 of this genus have a subterranean rhizome (stem), from which arise aerial stems 

 and true roots. The aerial stems are vertically grooved and jointed at the nodes 

 with a whorl of small leaves or branches at each node. 



