SAPROPHYTISM AND SYMBIOSIS 787 



relationship may be called one of allernalive parasitism. The term 

 helotism {i.e. slavery) may be employed in cases where one symbiont 

 derives appreciable nutritive benefit without conspicuous gain or loss 

 to the other. Thus helotism is intermediate between parasitism, where 

 one symbiont gains and the other loses, and commensalism, where 

 parasitism is absent. Where one symbiont lives within another, obtain- 

 ing food saprophytically rather than parasitically, it is said to exhibit 

 endosaprophytism. 



Root tubercles and their bacteria. — Structure and behavior. — Recip- 

 rocal parasitism is best illustrated by the relation existing between the 

 Leguminosae and the bacteria which inhabit galls on their roots ; these galls 

 are caused either directly or indirectly by the bacteria and are known 

 as root tubercles (fig. iioi). The tubercles, like most galls, are com- 

 posed chiefly of large parenchymatous cells, many of which, especially 

 in the central region, contain bacteria belonging to the species Bacillus 

 (or Pseudomonas) radicicola (fig. 1102). These bacteria are facultative 

 forms that are present somewhat generally in the soil as saprophytes, 

 appearing usually as minute motile rods. They enter the roots of legu- 

 minous plants through the root hairs, exhibiting prochemotactic reactions 

 to certain root excretions, and appearing to secrete substances which 

 soften or dissolve the walls of the root cells. Soon after the bacteria thus 

 become parasitic, there appear hypha-like infection threads or bacterial 

 tubes, which are essentially gelatinous masses of minute bacteria known 

 as zoogloea. The bacteria in these zoogloea masses bud like yeast, giving 

 rise to motile or immotile rodlike forms, which in turn produce the pecul- 

 iar branched forms, known as bacteroids,^ that are characteristic of the 

 tubercles. Finally, the organisms become dissolved and incorporated 

 into the body of the green plant. Tubercle formation is associated 

 definitely with bacterial infection, and starch may accumulate in the 

 hypertrophied cells as in other galls. 



While the root bacteria of the Leguminosae commonly are referred to a single 

 morphological species, there are several and perhaps many well-defined physio- 

 logical species. Commonly it is easy to infect the root of a legume with bacteria 

 from a legume of the same species or even genus, but usually it is difficult or impos- 

 sible to do this if the bacteria are taken from another genus; however, peas are 

 infected readily by bacteria from vetches, and vetches may be infected similarly 

 from peas. Bacteria from the roots of peas thrive only moderately in the roots of 



1 This name dates back to the time when the organisms were regarded as an albumi- 

 nous product of the green plant. The term Rhizobium was given to these bodies when 

 they were supposed to be organisms, but of unknown affinity. 



