EXTERNAL CAUSES OF GROWTH AND FORMATION. II 325 



it is not always easy to prove this. We have already gained some acquain- 

 tance with the stimulating effects of genuine parasites, but possibly the algal 

 constituents of lichens do not always benefit in that way ; at all events, we 

 notice one injurious result, viz. that they do not produce fruit when so 

 united. As to the form of the federation we may distinguish important differ- 

 ences. In many lichens, e.g. Ephebe, the alga is the predominant partner 

 and the lichen has essentially the appearance of the alga without the fungus. 

 Cora, on the other hand, one of the Hymenolichenes, has exactly the appearance 

 of the constituent fungus, one of the Telephoreae. In the majority of cases 

 the symbiotic union results in an entirely new form, with all the characters of a 

 single independent organism. In these cases obviously each organism influences 

 the other, and sometimes the one, sometimes the other, partner is predominant. 

 According to HOLLER'S (1893) researches, this is the case with Dictyonema, 

 a lichen the fungal constituents of which is the same species of Telephoreae which 

 occur in Cora. If the alga be a member of the Chroococcaceae, the lichen Cora 

 is the result, but a Dictyonema if it be a Scytonema. The filaments of Scytonema, 

 however, are much more vigorous than the unicellular Chroococcaceae, and thus 

 produce more arbitrary growth forms. The alga and the fungus carry on a sort 

 of struggle to determine the ultimate form of the compound organism. If the 

 lichen be developed in the air the fungus is the dominant partner (true Dic- 

 tyonema form), but if it be transferred to a solid support, the alga gains the 

 upper hand and it alone determines what the form of the compound organism 

 shall be (Laudatea form), and the fungus becomes merely an accompaniment 

 (MOLLER, 1893). The fungus can also live apart from the alga as a member of 

 the Telephoreae but may, when the suitable alga is met with, assume in the 

 course of its development either the Cora, Dictyonema, or Laudatea form. 



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