Kelley — 104 — Mycotrophy 



roots") while mycodomatia are often enlargements of the larger 

 ("mother") root, either of the root as a whole or in part, or of a sub- 

 terranean stem. Smaller excrescences are termed tubers, tubercles, 

 bulbs, etc., although the actual differences between them are not great 

 unless we consider the former as primarily lateral outgrowths. As to 

 the exact nature of nodules, there is obvious disagreement, but per- 

 haps it will be found that they are chiefly the result of a bacterial 

 stimulus and the fungi which are often if not always associated may 

 be associate commensals, if such an expression may be used. Again, the 

 exact nature of tubers, etc., is not altogether clear, but it seems that 

 they are ordinarily called forth by a fungal stimulus, although other 

 influences acting on the cell-plasm may equally well produce the tuber. 

 Of course a tuber is strictly a stem (as in tuber of potato) and bears 

 scale-leaves ; but the word is also freely applied to tuberous roots as 

 in the dahlia, and in both cases the enlarged growth appears due to 

 a fungus, hence the justice of the term, mycodomatium. In orchids, 

 the mycodomatia are sometimes true tubers, as in Aplectrum or 

 Tipularia, or are tubercles or enlarged roots. Bulbs and corms are 

 apparently also to be classed as mycodomatia. 



Psilotum: — Our information on Psilotiim infection is meagre, 

 but it is certain that Gallaud (1905) listed this plant in "Series 4" 

 with the orchids, and said it was similar in its mycotrophy with Tamus. 

 Psilotum is an "humus saprophyte" in which rootage organs are much 

 branched rhizomes that bear small gemmae on the subterranean shoots. 

 Yet Psilotum can grow asymbiotically as discovered by Bernard and 

 reaffirmed by Costantin (1925, 1936), who said that the plant had 

 been grown asymbiotically at the Museum at Paris for 132 years. 



Gycads: — The root tubercles of Cycas, according to Bottomley 

 (1907), are morphologically lateral roots showing a central vascular 

 cylinder with a well-marked endodermis completely surrounded by 

 "bacteroid" tissue. These tubercles are dichotomous and perennial, 

 and they differ from leguminous nodules, which are of limited growth. 

 Kellerman in 1910 isolated N-fixing bacteria from Cycas nodules, 

 while Life (1901) found in C. revoluta both bacteria and hyphae of 

 a fungus which resembled Rhizohium. These organisms, he said, are 

 confined to the mediocortex. Life thought the functions of the 

 tubercles were aeration and N-assimilation but decided it is difficult 

 to speak with certainty of the symbiotic relations of the various organ- 

 isms within. Spratt (1915) said that the tubercles are formed pri- 

 marily by Bacillus radicicola, and noted that in them four symbionts 

 are concerned, — two bacteria, an alga and the cycad. 



