A study of the root nodules of Myrica gale L

A study is made of the development and structure of the root nodules of Myrica gale utilising the water-culture technique for the growing of the plants. Observations are also made on field material. The general cultural methods are described and observations made on the germination and early stages of growth of the plant. From cytological evidence it is concluded that the organism responsible for nodulation is a member of the Actinomycetes and that it makes its way into the plant via the root hairs which become twisted and distorted in the process. Nodules can be observed some 14 days after inoculation, when the plat is at the 3 leaf stage, and they arise due to meristematic divisions of the parent root pericycle under the influence of anthocyanin, (not haemoglobin as in the nodules of the Leguminosae). It is characteristic of the nodules that each develops a terminal prolongation termed the "nodule-root" which proceeds to grow upwards. The nodule branches to give rise to fresh lobes which eventually form a close cluster enmeshed by the nodule roots. Internally the infected cells of the nodule become enlarged and the actinomycete can be seen within them. The actinomycete threads become swollen and are eventually digested sometimes forming a dark clump within the cells (in many ways similar to that seen in the digestive cells of the roots of Neottia.) Stages in digestion are described and figured. Attempts to isolate the causative organism are described. All of these have proved to be unsuccessful, the isolated organisms failing to induce nodulation. An examination is made of previous authors attempts at isolation and it is concluded, despite claims to the contrary, that the organism has not yet been isolated. A discussion follows on the possible reasons for failure and the evidence examined for the possibility of the organism being an obligate symbiont. Cross-inoculation experiments, using certain rhizobia and the endophytes of the root nodules of Alnus and Myrica, are described. From these it is concluded that none of the three are cross inoculable. A comparison is made of the effect of the pH of the rooting medium on nodule formation in two non-legums (Nyrica gale and Alnus glutinosa) with that in a typical legume, namely Trifolium pratense, utilising data obtained by Bond (1951) on Myrica gale and unpublished data on Alnus glutinosa kindly communicated to the author by Mr. T. Ferguson of the Botany Department, Glasgow University. In order to provide a direct comparison, the author has obtained data from Trifolium pratanse by growing it, as were the two non-legumes, in water culture at pHs ranging from 3.3-7.0 It is shown that the endophytes of all three plants have different pH tolerances - the endophyte of T. pratense cannot survive a pH of 4.2 or lower; the endophyte of A. glutinosa can survive a pH of 4.2 but not of 3.3 or lower. The endophyte of M. gale can survive a pH of 3.3 It is further demonstrated for M. gale and A. glutinosa that the reaction of root and nodule tissue does not vary with the pH of the medium in which the plants are growing. In M. gale the root pH is 5.0-5.3 and the nodules pH 5.4-5.8 over a solution pH range of 4.2-7.0 In A. glutinosa the root pH is 4.9-5.2 and the nodule pH 5.8-6.0 over a solution pH range of 4.2-7.0 The effect of combined nitrogen (as NaNO3 and (NH4)2SO4) on nodulation in Myrloa gale is investigated, together with comparative data for Trifolium pratense, and it is shown that whereas a small quantity of combined nitrogen (17.5 mgms. N/litre) markedly depresses nodulation (both as regards size and number of nodules) in Trifolium combined nitrogen (up to 140 mgms. N/litre) does not affect the number of nodules found on Myrica; and that the nodules of Myrica plants receiving combined nitrogen increase in size much more rapidly than those developing in nitrogen free solution. It is concluded that the effect of combined nitrogen free solution. It is concluded that the effect of combined nitrogen on nodulation is fundamentally different in the two plants and the Myrica-actinomycete symbiosis is a less highly evolved system than the legume-rhizobium association since in the former there is a superflucus expenditure of raw materials in the formation of nodules in the presence of combined nitrogen