248 



PLANT MORPHOLOGY 



genera, Hyenia and Calamophyton, both of which Hved during the middle 

 Devonian. The aerial shoots were slender and dichotomously branched 

 and, in one species of Hyenia, are known to have arisen from a stout 



horizontal rhizome. The stems 

 were jointed in Calamo'phyton but 

 not in Hyenia (Fig. 205). The 

 leaves were small, narrow, and 

 whorled. In Hyenia they were 

 forked several times, in Calamo- 

 'phyton forked only once. Little 

 is known of the vascular anatomy. 

 The stem of Calamo'phyton was 

 apparently siphonostelic and is 

 thought to have undergone some 

 secondary thickening. 



The sporangia of the Hyeniales 

 were borne on sporangiophores 

 that were grouped to form a loose 

 strobilus in which no bracts were 

 present. In both genera the spo- 

 rangiophores were once forked, the 

 tip of each division being recurved 

 and bearing two or three pendent 

 sporangia in Hyenia, but only one 

 sporangium in Calamophyton. 

 Presumably the Hyeniales were 

 homosporous. 



A 



Fig. 205. Calamophyton primaevum. A, 

 reconstruction of aerial shoot; B, sterile 

 leaves; C, sporangiophores. (After Krausel 

 and Weyland.) 



2. Sphenophyllales 



This is an order of Paleozoic 

 plants ranging from the Devonian 

 to the Triassic. In many respects 

 it is intermediate between the 

 Lycopodiinae and Equisetinae; in fact, it is often considered as a separate 

 class of pteridophytes. There are three important genera: Spheno- 

 phyllum, Cheirostrobus, and Pseudobornia. 



The slender fluted stem bore whorls of leaves separated by elongated 

 internodes, with branches arising at the nodes (Fig. 206). The leaves, 

 usually six at a node, were mostly simple and wedged-shaped, but were 

 often dichotomously divided into narrow lobes, while in Pseudobornia 

 fern-like leaflets were present. The stem, m all cases, was a protostele 

 with exarch xylem and a considerable amount of secondary thickening. 

 The strobili were terminal and composed of whorled sporophylls show- 



