ROOT TUBERCLES OF OTHER PLANTS 263 



number of other families, though not always in all their 

 members (cp. Kellerman, 1910). These are the Cycadaceae, 

 Podocarpaceae, Eleagnaceae (Eleagnus, the oleaster, Hippo- 

 haes, the sea buckthorn, and Shepherdia), Rhamnaceae 

 {Ceanothus americaniis, and C. veliitinus), Myricaceae 

 {Myrica Gale, the bog myrtle or gale, and M. asplemfolia), 

 Betulaceae (Alnus, the alder), and perhaps Casuarina. The 

 nature of the organism in some of these cases is doubtful. 

 Peklo (191 o) assigns the symbionts in Myrica and Alnus to 

 the peculiar bacterial genus Actinomyces. Shibata (1902) 

 regards the nodules of Podocarpus as mycorhiza. Moeller 

 (1890) describes the symbiont of Alnus as a non-septate 

 fungus. Nodules on Casuarina have been described by 

 Miehe (191 8) as a case of mycorhiza. Bottomley (191 2 a 

 and b, 1915) and Spratt (i9i2« and b, 1915) have, however, 

 isolated the organism of Alnus, Myrica, and Eleagnus, 

 Ceanothus, Podocarpus and Cycas. They state that in 

 all cases it is a form of Bacillus radicicola. All these plants 

 are capable of thriving in nitrogen-free media if infected with 

 the appropriate organism, which fixes free nitrogen. 



In the non-leguminous plants the bacillus infects the 

 root through the root hairs, passes into the cortex and there 

 infects a young lateral root as it passes outwards through 

 the tissues (Fig. 35). This lateral root then grows out and 

 becomes hypertrophied, producing the mass of parenchyma 

 which is inhabited by the symbiont. Except in the Podocar- 

 paceae the lateral root forks repeatedly, and the tubercle 

 assumes a more or less coralloid form (cp. Fig, 34). These 

 tubercles are perennial (a condition confined in the 

 Leguminosae to the tribe Mimoseae), and, as growth 

 continues year after year, they may assume large dimensions. 

 In the alder they may be as big as a cricket ball. 



Especially interesting are the two gymnospermous 

 famiHes the Podocarpaceae and Cycadaceae, in all the genera 

 of which so far examined tubercles have been found. The 

 tubercles of the Podocarpaceae are simple, and the fine roots 

 beset with them have the appearance of a loosely threaded 

 string of beads (cp. Fig. 34). The conditions in the Cycadaceae 



