Cell walls mycobiont

The hyphal cell walls of three mycobionts isolated from the lichens Xanthoria parientina, Tornabenia intricata, and Sarcogyne sp. are composed of chitin and glucan. Fluorescein-labelled lectins were used to locate the polysaccharides in the cell wall. 

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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