Cell walls phycobiont

Lichen substances affect the permeability of the cell walls of the phycobionts, and could play an important role in symbiotic equilibrium (FoUmann and Villagran 1965 Kinrade... 

Recent observations of Lichina pygmaea indicate that special structures of the algae also may favor a transport of metabolic products (Peveling, 1973). The plasmalemma and outer cell wall of the Calothrix phycobiont can evaginate. At the same time numerous small vesicles appear in the fibrillar sheath of this blue-green symbiont. 

Jacobs and Ahmadjian (1969), who examined the ultrastructure of ten foliose and fruticose nongelatinous lichens, found a thick (up to 1 /urn) fibrillar material of a polysaccharide nature surrounding the fungal cell wall. A similar extracellular polysaccharide was also found between the cells of the Trebouxia phycobiont. This substance supposedly facilitates the water retention ability of the thallus. 

In the cycad symbiosis, Nostoc filaments penetrate fractures in the root cell walls to form an intracortical phycobiont layer similar in form to that of heteromerous lichens. It is conceivable, therefore, that the blue-green phycobiont might become incorporated in the mycobiont hyphae even though the present morphology of the phycobiont cells seems to preclude this. They are not known to be motile within the thallus and are usually much enlarged in size and aggregated compared with the motile free-living condition. 

The mechanical construction of the green lichen thallus is more compatible with the incorporation of the mycobiont hyphae within the phycobiont cells—appressoria and haustoria being of frequent occurrence. This is the normal parasitic trend in which penetration of the host is achieved by enzymatic dissolution and mechanical rupture of the host cell wall by growth in length of the parasite hyphae. 

There are thus two opposing trends to be seen within the green lichens at the present time. The parasitic tendency of the mycobiont hyphae to penetrate the phycobiont cells can be viewed as a retrogressive feature in that it tends to preserve the status of parasitism rather than symbiosis. On the other hand, the tendency toward elimination of the phycobiont cell wall and increase in membrane permeability can be viewed as a progressive step if we can legitimately consider such trends to be homologous to those evident in other symbiotic systems. 

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