lichens: their economic role. 259 



logical units rather than symbiotic colonies of algae and fungi 

 growing together. 



But it were folly for us to enter at length into a controversy 

 that has filled volumes. Whether the lichen may be regarded as a 

 plant in any true sense or not, we may speak of the " lichen plant " 

 in order to avoid unnecessary circumlocution. Also, certain 

 mycologists consider witches' brooms, burls, swellings, due to 

 fungus infection, etc., as individuals, at least in a physiological 

 sense, and if this view is well founded, lichens have surely added 

 to definite function constancy of morphological characters and 

 may as reasonably be considered autonomies, both physiological 

 and morphological. 



While we may not regard as settled the question of the nature 

 of lichens, there are many questions regarding the morphology 

 and physiology of these, at least so-called plants, that are of very 

 great interest to the biologist, and it is worth while to recall the 

 relation of lichens to other things in nature from the point of view 

 of utility or detrimental efl:ect. In giving this popular statement, 

 the present writer can do no more than bring together once more 

 some of the facts that have repeatedly appeared in print. 



Lichens, like the higher chlorophyll-bearing plants, take large 

 amounts of COo from the air in the processes of nutrition ; they 

 build up lichenin, a carbon compound very similar to starch, and 

 return to the atmosphere as free oxygen the portion not needed 

 in the production of the lichenin. It is estimated that the higher 

 plants take half of their food thus from the air and the other half 

 from the earth through the roots. It was formerly supposed that 

 lichens took a very small proportion, if any, of their food from 

 the substrata ; but that view is certainly incorrect, at least regard- 

 ing many lichens. However, it is quite likely that most lichens 

 take a smaller proportion of their food from the substratum, and a 

 larger proportion from the air than do the higher chlorophyll- 

 bearing plants. The algal cells of the lichen-partnership do the 

 work of building up lichenin, while the fungal portion of the 

 lichen furnishes protection to these algae and also takes more or 

 less of crude or elaborated food materials from the substratum. 

 Thus the lichens, in the ordinary processes of nutrition, purify 



