Kelley — 24 — Mycotrophy 



tions which exist between these various organisms it is difficult to 

 speak with any certainty . . . the tubercles of Cycads may be said to 

 have at least two functions, that of aerating and that of assisting nitro- 

 gen assimilation." But whatever their structure or function, they can 

 be considered only as a very special case of mycorrhiza, and the same 

 may be said for the "nodules" of Encephalartos; while those of 

 Macrosamia are reported by McLuckie (1922) to be purely bacterial. 

 Spratt (1915), whose work on Cycadean nodules comes nearest to 

 being monographic, states that all Cycadean genera produce root- 

 nodules which are perennial modified lateral roots, repeatedly branched 

 and forming large coralloid masses. They are primarily produced, he 

 says, by infection with Bacillus radicola ; and he asserts that the Cyca- 

 deaceae are the only nodule-bearing plants known in which four 

 organisms are associated together symbiotically, viz. two nitrogen- 

 fixing bacteria, an alga, and the cycad. 



Mycorrhizae of Ginkgo: — Only one living member of this genus 

 occurs and this member, the Maidenhair tree, has long since ceased to 

 exist in a wild state. It is thus in the nature of an exotic wherever it 

 grows, and its rooting conditions and structures are in a sense anoma- 

 lous. Perhaps no other plant, the lone representative of its order, 

 presents such an unique case ; yet Ginkgo is reported mycorrhizal. Its 

 earliest observer was Reinke (1873) who noted "thickening strips" 

 in its root cortex ; its latest observers were Klecka and Vukolov 

 (1935) who state that the mycorrhizae are racemose, slightly furcate. 

 Yet ScHWARZ (1883) and von Tubeuf (1896) reported abundant 

 root-hairs for this species. An ecological study of Ginkgo roots in the 

 native haunts of the species, so far as China could provide "native 

 haunts", would be desirable. 



Mycorrhizae amongst the Taxaceae: — The curious tubercles 

 and necklace rootlets of various Podocarpi have proved fascinating 

 to students of root structure. There is something which arouses 

 curiosity in them : the roots are excavated, some lumpy excrescences 

 appear, and forthwith the botanist hurries to his laboratory to see what 

 meaneth this strange thing! A mere ordinary rootlet is passed by as 

 commonplace : for example, the possible mycorrhizae of Torreya are 

 virtually unknown, apparently because there is nothing about them 

 to attract curiosity. But thanks to studies of the curious we have much 

 on record about the nodules of ten species of Podocarpus. It appears 

 that these nodules are called forth by bacterial action as well as by 

 fungal invasion ; but the consensus of opinion seems to be that they 

 are often true mycorrhizae, being developed usually by a symbiotic 



