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LICHEN GONIDIA 21
i. GONIDIA IN RELATION TO THE THALLUS
A. HISTORICAL ACCOUNT OF LICHEN GONIDIA
There have been few subjects of botanical investigation that have
roused so much speculation and such prolonged controversy as the question
of these constituents of the lichen plant. The green cells and the colourless
filaments which together form the vegetative structure are so markedly
dissimilar, that constant attempts have been made to explain the problem
of their origin and function, and thereby to establish satisfactorily the
relationship of lichens to other members of the Plant Kingdom.
In gelatinous lichens, represented by Collema, of which several species
are common in damp places and grow on trees or walls or on the ground,
the chains of green cells interspersed through the thallus have long been
recognized as comparable with the filaments of Nostoc, a blue-green
gelatinous alga, conspicuous in wet weather in the same localities as those
inhabited by Collema. So among early systematists, we find Ventenat 1
classifying the few lichens with which he was acquainted under algae and
hazarding the statement that a gelatinous lichen such as Collema was only
a Nostoc changed in form. Some years later Cassini 2 in an account of Nostoc
expressed a somewhat similar view, though with a difference: he suggested
that Nostoc was but a monstrous form of Collema, his argument being that,
as the latter bore the fruit, it was the normal and perfect condition of
the plant. A few years later Agardh 3 claimed to have observed the meta-
morphosis of Nostoc up to the fertile stage of a lichen, Collema limosum.
But long before this date, Scopoli 4 had demonstrated a green colouring
substance in non-gelatinous lichens by rubbing a crustaceous or leprose
thallus between the fingers; and Persoon 5 made use of this green colour
characteristic of lichen crusts to differentiate these plants from fungi.
Sprengel 6 went a step further in exactly describing the green tissue as
forming a definite layer below the upper cortex of foliaceous lichens.
The first clear description and delimitation of the different elements
composing the lichen thallus was, however, given by Wallroth 7 . He
drew
attention to the great similarity between the colourless filaments of the
lichen and the hyphae of fungi. The green globose cells in the chlorophyllaceous
lichens he interpreted as brood-cells or gonidia, regarding them as
organs of reproduction collected into a "stratum gonimon." To the same
author we owe the terms "homoiomerous" and "heteromerous," which he
coined to describe the arrangement of these green cells in the tissue of the
thallus. In the former case the gonidia are distributed equally through the
structure; in the latter they are confined to a distinct zone.
1 Ventenat 1794, p. 36.
5 Persoon 1794, p. 17.
2
Cassini 1817, p. 395.
3
Agardh 1820.
4
Scopoli 1/60, p. 79.
6
Sprengel 1804, p. 325.
7 Wallroth 1825, I.