IRREGULAR NUTRITION 



235 



where, however, the parasitism is of a milder type. The two organisms, 

 the fungus and the higher plant, essentially antagonistic to each other, 

 may be looked upon as being in a state of balance ; and that this 

 leads to a prolonged symbiotic existence together, from which both 

 may gain advantage. In the above treatment the possible benefit 

 to the higher plant has received most attention, but the fungus 

 probably gains at least a favourable en- 

 vironment, and some food materials from 

 the higher plant. 



The symbiotic condition presented in 

 mycorrhiza occupies a middle position be- 

 tween two extremes. The one is that of 

 mortal disease, where one organism of an 

 association causes the ultimate death of 

 the other. The other extreme is that of 

 immunity, where though two organisms may 

 be in relation, the one has no power over 

 the other. There is evidence that under 

 conditions unfavourable to the development 

 of the higher plant the state of balance 

 in the mycorrhiza may be upset, and that 

 the fungus develops more pronounced 

 parasitic tendencies. 



Root-Nodule Plants. 



Plants of the family Leguminosae (which 

 include Peas, Beans, Clovers and Lupins) 

 are regularly characterised by the presence 

 of nodules on their roots. The nodules yield 

 another example of Symbiosis, in this in- Root of ^fL, ^ nu . 

 stance between a flowering plant and a bac- ™ e /° us c /°°K nod ^ es - Reduced - 



r (After Strasburger.) 



terium. The lower organism is again perhaps 



best regarded as being essentially parasitic in nature. It inhabits the 

 cells not of the root itself, as in endotrophic mycorrhiza, but of those 

 special structures, the nodules, which arise on the roots and which 

 may be compared with other swellings produced on plants by the 

 presence of a foreign organism, e.g. galls. 



The nodules may be either spherical or cylindrical, simple or 

 branched, according to the particular plant concerned, and they are 

 present on main and lateral roots (Fig. 163). The nodule consists of 



