Hedlund followed the process of association between the two organisms in the lichens _Micarea_ (_Biatorina_) _prasina_ and _M. denigrata_ (_Biatorina synothea_), 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 contents into two or at most four portions which become rounded off and covered with a membrane 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_. [Illustration: Fig. 11. Gonidia from _Ramalina reticulata_ Nyl. A, gonidium pierced and cell contents shrinking × 560; B, older stage, the contents of gonidium exhausted × 900 (after Peirce).] [Illustration: Fig. 12. _Pertusaria globulifera_ Nyl. Fungus and gonidia from gonidial zone × 500 (after Darbishire).] Penetrating haustoria were demonstrated by Peirce[213] in his study of the gonidia of _Ramalina reticulata_. 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. 11). More advanced stages showed the extension of the haustorium into the centre of the cell, and, finally, the 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[214] 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. Lindau[215] has described the association between fungus and alga in _Pertusaria_ and other crustaceous forms as one of contact only (Fig. 12). He found that the cell-membrane of the two adhering organisms was unbroken. Occasionally the algal cell showed a slight indentation, but was otherwise unchanged. The hyphal branch was somewhat swollen at the tip where it touched the alga, and the wall was slightly thinner. The attachment between the two cells was so close, however, that pressure on the cover-glass failed to separate them. Generally the hypha simply surrounds the gonidium with clasping branches. Many algae also lie free in the gonidial zone, and Peirce[216] claims that these are larger, more deeply coloured and in every way healthier looking than those in the grasp of the fungus. He ignores, however, the case of the soredial algae which though very closely invested by the fungus are yet entirely healthy, since on their future increase depends in many cases the reproduction of new individual lichens.