204 BOTANY OF THE LIVING PLANT 



is realised in nature when two antagonistic organisms, by balancing 

 their powers, attain to a life of mutual toleration lasting for a prolonged 

 period, as in Lichens, or mycorhizic Flowering Plants. The number 

 of such associations now recorded has rapidly increased in late years, 

 while varying degrees of dependence, and of toleration or obligation 

 have been detected. 



It appears further that there may be variation from time to time in 

 the aptitude of the attacking organism for living with its host. In 

 the case of Rhizoctonia it has been shown that by an autonomous 

 life it gradually loses its power, or even becomes incapable of causing 

 the seeds of the Orchid to germinate. On the other hand, it increases 

 that power by living with its host. This varying aptitude for sym- 

 biosis seems in every way comparable to the varying virulence of 

 pathogenic organisyns. Similar variations in the power of attack were 

 demonstrated long ago by de Bary in the case of Sclerotinia, and other 

 cases are known among pathogenic Fungi. 



Both in ectotrophic and in endotrophic mycorhiza various steps 

 are illustrated between partial and complete physiological dependence. 

 Where the dependence is partial there is little or nothing in the 

 appearance of the plants above ground to suggest irregular nutrition ; 

 excepting in some cases paleness of colour, which indicates imperfect 

 development of chlorophyll. The complete result is shown on the 

 one hand by Monotropa or Sarcodes, on the other by Corallorhiza or 

 Neottia. In both cases chlorophyll is absent. There must then have 

 been an indirect supply of organic material from the humus outside. 

 The fungus supplies the intermediate step. It thus appears that both 

 types of mycorhiza culminate in complete Saprophytism at second hand. 



Root-Tubercles. 



The formation of tubercles upon roots, and their importance in the 

 fixation of Nitrogen has already been mentioned in Chapter VII. p. io6. 

 Such tubercles are found almost universr.lly in the Leguminosae, and 

 similar growths are also known in the roots of other plants, such as 

 the Alder Tree. It will be seen that these tubercles have much the 

 same physiological significance as mycorhiza, and like it involve 

 digestion of the intrusive organism. 



In any ordinary crop of Beans the roots are found to bear numerous 

 tubercular swelhngs, often of large size and irregular outline (Fig. 155). 

 Dissection shows that they are seated just outside the stele, and may 

 when young be mistaken for lateral roots, since they arise from the 



