Lecture IV — 51 — Fossil Mycorrhizae 



the Lower Coal Measures, it is not unusual to meet with fragments of 

 fungal threads. Peronosporites gracilis is very widely distributed in 

 this horizon, the hyphae occurring in cortex of young rootlets of 

 Lygiuodendron Oldhamium and are not wanting in the stele, in which 

 both hyphae and vesicles were found. The fungus was probably a 

 parasite according to Ellis. He says further that vesicles, both 

 terminal and intercalary, were found, and tuberous swellings. While 

 in older plants the cortex alone is invaded, in young plants stelar 

 cells are also infected. 



Mycorrhizae in Gordaites: — One tree at least of the Palaeozoic 

 was provided with mycorrhizae, for these structures in Cordaites 

 have been described in some detail. Osborn (1909) described the 

 roots of Amyclon radicans, which has been shown to belong to this 

 group. It bears such remarkable and irregularly arranged bunches of 

 lateral roots that Osborn examined them to discover if these bunches 

 might correspond in any way with the root tubercles of recent plants. 

 These lateral roots are found to have a thick cortex divisible into 

 two regions, the inner of which contains dark cells that show evident 

 fungal hyphae. The fungus occurs in knots of non-septate hyphae 

 that sometimes bear terminal vesicles but there was no trace of spore 

 formation. The conclusion was reached that this tree probably in- 

 habited saline swamps and had bunches of coralline roots such as are 

 known to occur in many recent plants under similar conditions. 

 Osborn considered the relation of fungus to Amyelon to be in the 

 nature of a mutualistic symbiosis. 



In another study of Amyelon, Halket (1930) made sections of 

 a British coal-ball and found in longitudinal sections of rootlets of 

 Amyelon that they "not only showed the structure of the root-cortex 

 but also had fungal hyphae present in its cortex, and forming a definite 

 'fungal zone' round the stele". The description and excellent photo- 

 micrographs indicate that the structure of those ancient Carboniferous 

 rootlets was very similar to that of coniferous rootlets of today. Root- 

 hairs were not as a rule developed. The diarch rootlets, which 

 branched laterally as a result of division of cells in the pericycle, had 

 apices which indicated that many of the rootlets had "limited growth". 

 The (septate) hyphae were mainly intercellular but formed vesicles 

 and arbuscles intracellularly. The author mentions, and the illustra- 

 tions would seem definitely to> indicate, digestion stages in cortical 

 cells; but the vascules were never invaded. Halket considered the 

 symbiosis to be of mutual benefit. 



