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stratified, as it were, between different layers of the remaining 
tissue ; in the rest no such stratification exists, but the green cells 
are distributed throughout the whole thallus, or in a few instances 
seem to form the greater part of it. 
In every case, whether stratified or unstratified, the coloured 
cells seem to lie loose and unattached among the other anatomical 
elements. This want of connexion at first gave rise to the opinion 
that they were asexual reproductive organs, analogous in some 
degree to the gemme and bulbils of higher plants, and accordingly 
the name gonidia was given to them. The supposed function of 
reproduction attributed to them seemed to early botanists to 
explain everything that was necessary respecting them, and for a 
long time no inquiry as to their mode of origin was entered upon. 
At length a French botanist, Tulasne, conducted a very skilful 
series of researches into the nature of various organs of Lichens 
and Fungi. In the course of his enquiry he sowed some Lichen 
spores, and watched their development over a space of many 
months. He saw that the spore put out filamentous cells which 
branched again and again. At certain points on these cells he 
discovered that others of a different order arose, and he watched 
their development till gradually a tissue was formed which produced 
in its interior chlorophyllous cells like those of mature plants. Not 
having his work at hand while I write, I am uncertain whether the 
plants lived to produce perfect thalli, but I believe they did not. 
However, he had seen enough to convince himself that the green 
cells—the gonidia—were produced from the other primitive tissue 
by a process of differentiation. 
Dr. Nylander, to whom I have already referred as the greatest 
living authority on Lichens, took up the question of the origin of 
these green cells with reference to their production in later stages 
of the life of the plant. His examination proved to him that they 
arose amongst the hyaline cells which lie just above the gonidial 
layer—I am referring to stratified thalli-or in that part of the 
plant where its vital activity is greatest, that at first they were in 
direct connexion with the tissue which produced them, and that it 
