Lecture II — 25 — Occurrence of Mycorrhizae 



fungus although in cases a fungus is lacking. Reinke (1873), who 

 saw much and described well, noted "thickening strips" in Podocarpus 

 cortex; Berggren (1887), with the meticulous exactness of a 

 Scandinavian, described in detail the pearl-necklace rootlets of the 

 Podocarpineae, seeing in them a similarity in function to the velamen- 

 covered roots of orchids. The similarity cannot be drawn too closely, 

 however, for Hiltner (1899), in conjunction with Nobbe, demon- 

 strated fixation of atmospheric N by roots of Podocarpus, which he 

 considered as true endotrophic mycorrhizae ; yet Hiltner suggests 

 that Heaths and Orchids likewise may fix nitrogen. McLuckie 

 (1923) also found the Podocarpineae active in N fixation, stating 

 that the process was accomplished by bacteria present in the cortical 

 cells. On the other hand, Saxton (1930) was unable to find bacteria 

 in Tasmanian material of Podocarpus : "No trace of bacteria could be 

 found but unmistakable and well-preserved mycorrhizal filaments." 



Hiltner had considered these nodules as unformed roots but 

 McLuckie (1923) says that the nodules are modified lateral roots 

 and arise from the pericycle, their normal growth being checked before 

 they emerge from the cortex of the main root. Root-hairs, he says, are 

 commonly present as von Tubeuf had already stated. Yet it is neces- 

 sary to be careful about accepting reports of root-hairs on mycorrhizal 

 roots too readily, for setae of the fungus often simulate root-hairs ; and 

 McLuckie himself says that the surface of the nodule and the main 

 root is frequently invested with a loose tangle of fungal hyphae. 



It is interesting to note that the term "prosporidi" of Petri (1903) 

 was originated from a study of species of Podocarpus growing at 

 Florence in Italy. These spore-like bodies produced by the fungus, the 

 sporangioles of Janse, he called "prosporidi" on account "al loro 

 significato morphologico piu probabile". Shibata (1902) described 

 in some detail the fungal structure and reaction, and reputedly demon- 

 strated an enzyme in the mycorrhiza. 



In addition to the nodules of Podocarpus are the mamillate or 

 pearl-necklace rootlets described so well by Janse (1897). In P. 

 cupressus he found intermittent growth : "En general, apres une 

 courte interruption, la croissance reprend pour s'arreter encore une 

 fois des qu'il s'est forme en second mamelon spherique au sommet du 

 premier. Cette croissance intermittente pent se repeter ainsi plusieurs 

 fois de suite, mais au plus tard apres le developpement du cinquieme 

 mamelon I'arret est definitif ." Janse continues with a detailed descrip- 

 tion of the histological structure and origin of these mamelons or 

 pearl-necklace mycorrhizae which are so widely found amongst coni- 

 fers, casuarinas, Liquidamhar, Acer, Celtis, and others. 



