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^ 

 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^ 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' claimed to have observed the meta- 

 morphosis of Nostoc up to the fertile stage of a lichen, Collema limosztm. 

 But long before this date, Scopoli^ had demonstrated a green colouring 

 substance in non-gelatinous lichens by rubbing a crustaceous or leprose 

 thallus between the fingers; and Persoon^ made use of this green colour 

 characteristic of lichen crusts to differentiate these plants from fungi. 

 SprengeP 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'. 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 chlorophylla- 

 ceous 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. 



^ Ventenat 1794, p. 36. '' Cassini 1817, p. 395. * Agardh 1820. ^ Scopoli 1760, p. 79. 

 ° Persoon 1794, p. 17. " Sprengel 1804, p. 3'25. '' Wallroth 1825, I. 



