212 PLANT MORPHOLOGY 



being most common. Most fern stems have mesarch amphicribral bun- 

 dles. The characteristic stem of seed plants is an ectophloic sipho- 

 nostele with endarch collateral bundles, a type that is uncommon in 

 pteridophytes. 



Traces and Gaps. In all vascular plants the conducting sj^stem is 

 essentially continuous throughout the plant body. A strand of conduct- 

 ing tissue extending from the stele of the stem through the cortex to a leaf 

 is called a leaf trace. A strand connecting the vascular tissue of the stem 

 with that of a branch is a branch trace, while a root trace occurs where a 

 root arises from a stem or from another root. A gap is a break or inter- 

 ruption in a siphonostele caused by the departure of a leaf trace or a 

 branch trace (Fig. 221). It consists of parenchyma. Gaps are not 

 formed by root traces. Branch gaps are present in all siphonostelic 

 stems, but leaf traces occur only in ferns and seed plants. 



1. PSILOPHYTINAEi 



The Psilophytinae comprise a group of primitive pteridophytes that 

 were abundant and widespread during the Devonian, but are represented 

 today by only two genera of somewhat restricted distribution (Fig. 258). 

 They are all rootless plants and either leafless or provided with small, 

 simple, spirally arranged leaves. The sporangia are solitary and terminal 

 on branches that are elongated or, in modern forms, greatly reduced. 

 The Psilophytinae comprise two orders, the extinct Psilophytales and the 

 existing Psilotales. 



1. Psilophytales 



The Psilophytales are the oldest and most primitive of all known vas- 

 cular plants. They appeared in the late Silurian and flourished during 

 the early and middle Devonian. Their remains have been found in many 

 parts of the world, but the best-preserved material has come from Scot- 

 land. The chief genera are Rhynia, Hornea, Psilophyton, and Asteroxylon. 



The Psilophytales were small herbaceous plants that lived on land. 

 Few exceeded 60 cm. in height. The sporophyte consisted of a rhizome 

 bearing slender, erect, dichotomously branched shoots (Fig. 169). The 

 stems of Asteroxylon were covered with small simple leaves, but the three 

 other genera were leafless. In some species of Psilophyton the stems were 

 spiny. Apparently true roots were not present, but in some genera the 

 rhizome bore numerous rhizoids. In Psilophyton the tips of the branches 

 were circinately coiled, as in the young leaves of ferns. In all members 

 of the group the stem was a protostele, a narrow zone of phloem enclosing 

 a central mass of xylem. Around the stele was a wide cortex surrounded 

 by a cutinized epidermis with typical stomata. The presence of stomata 



1 Also called Psilopsida. 



