LICHEN GONIDIA 



33 



undamaged, is excited to a division which takes place on a plane that passes 

 through the haustorium; the two daughter-cells then separate, and in so 

 doing free themselves from the hypha. 



Hedlund followed the process of association between the two organisms 

 in the lichens Micarea (Biatorina) prasina and 

 M. denigrata {Biatorina synotkea), crustaceous 

 species which inhabit trunks of trees or palings. 

 In these the alga, one of the Chlorophyceae, has 

 assumed the character of a Gloeocapsa but on 

 cultivation it was found to belong to the genus 

 Gloeocystis. The cells are globose and rather 

 small ; they increase by the division of the con- 

 tents into two or at most four portions which 

 become rounded off and covered with a mem- 

 brane before they become free from the mother- 

 cell. The lichen hypha, on contact with any one 

 of the green cells, bores through the outer membrane and swells within to a 

 haustorium, as in the gonidia of Synalissa. 



Penetrating haustoria were demonstrated by Peirce' in his study of the 

 gonidia of Ramahna reticidata. In the first stage the tip of a hypha had 

 pierced the outer wall of the alga, causing the protoplasm to contract away 

 from the point of contact (Fig. ii). More advanced stages showed the 

 extension of the haustorium into the centre of the cell, and, finally, the 



Fig. 10. Synalissa syrnphorea Nyl. 

 Algae [Gloeocapsa) with hyphae 

 from the internal thallus x 480 

 (after Boriiet). 



Fig. 1 1 . Gonidia from Ramalina reticulata 

 Nyl. A, gonidium pierced and cell con- 

 tents shrinking x 560 ; B, older stage, 

 the contents of gonidium exhausted x 900 

 (after Peirce). 



Fig. 12. Pertiisaria globulifera'HyX. Fungus 

 and gonidia from gonidial zone x 500 

 (after Darbishire). 



complete disappearance of the contents. In many cases it was found that 

 penetration equally with clasping of the alga by the filament sets up an 

 irritation which induces cell-division, and the alga, as in Synalissa, thus 

 becomes free from the fungus. Hue- has recorded instances of penetration 

 in an Antarctic species, Physcia puncticulata. It is easy, he says, to see the tips 

 of the hyphae pierce the sheath of the gonidium and penetrate to the nucleus. 



' Peirce 1899. ^ Hue 1915. 



S.L 3 



