STRUCTURES PECULIAR TO LICHENS 151 



B. Origin and Function of Isidia 



Nilson^ (later Kajanus^) insists that isidia and soredia are both products 

 of excessive moisture. He argues that lichen species, in the course ofTiheir 

 development, have become adapted to a certain degree of humidity, and, if 

 the optimum is passed, the new conditions entail a change in the growth 

 of the plant. The gonidia are stimulated to increased growth, and the 

 mechanical pressure exerted by the multiplying cells either results in the 

 emergence of isidial structures where the cortex is unbroken, or, if the 

 cortex is weaker and easily bursts, in the formation of soralia. 



This view can hardly be accepted ; isidia as well as soredia are typical 

 of certain species and are produced regularly and normally in ordinary 

 conditions; both of them are often present on the same thallus. It is not 

 denied, however, that their development in certain instances is furthered 

 by increased shade or moisture. In Evernia furfuracea isidia are more 

 freely produced on the older more shaded parts of the thallus. Zopf ^ has 

 described such an instance in Evernia olivetorina {E. furfuracea), which 

 grew in the high Alps on pine trees, and which was much more isidiose 

 when it grew on the outer ends of the branches, where dew, rain or snow 

 had more direct influence. He^ quotes other examples occurring in forms 

 of E. furfuracea which grew on the branches of pines, larches, etc. in a damp 

 locality in S. Tyrol. The thalli hung in great abundance on each side of 

 the branches, and were invariably more isidiose near the tips, because 

 evidently the water or snow trickled down and was retained longer there 

 than at the base. 



Bitter' has given a striking instance of shade influence in Unibilicaria, 

 He found that some boulders on which the lichen grew freely had become 

 covered over with fallen pine needles. The result was at first an enormous 

 increase of the coralline isidia, though finally the lichen was killed by the 

 want of light. 



Isidia are primarily of service to the plant in increasing the assimilating 

 surface. Occasionally they grow out into new thallus lobes. The more 

 slender are easily rubbed off, and, when scattered, become efficient organs 

 of propagation. This view of their function is emphasized by Bitter who 

 points out that both in Evernia furfuracea and in Umbilicaria pustulata 

 other organs of reproduction are rare or absent. Zopf ^ found new plants 

 of Evernia furfuracea beginning to grow on the trunk of a tree lower 

 down than an old isidiose specimen. They had developed from isidia which 

 had been detached and washed down by rain. 



' Nilson 1903. ' Kajanus (Nilson) 191 1. ^ Zopf 1903. '' Zopf 1905^. 



" Bitter 1899. 



