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LICHEN GONIDIA
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 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 mothercell.
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.
Fig. 10. Synalissa symphorea Nyl.
Algae (Gloeocapsa) with hyphae
from the internal thallus x 480
(after Hornet).
Penetrating haustoria were demonstrated by Peirce 1 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
1 1 .
Fig. Gonidia from Ramalina reticulata
Nyl. A,gonidium pierced and cell contents
shrinking x 560 ; B, older stage,
the contents of gonidium exhausted x 900
(after Peirce).
Fig. 12. Pertusaria globultfera Nyl. 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 2 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.
1 Peirce 1899.
z Hue 1915.
S.L 3