Thallogen?.] LICHENALES. 49 



hirta and plicata, Parmelia fraxinea and farinacea, and various others, M. Kuop has 

 obtained a substance called Usnine or Usnic acid. This author finds the sulphur-coloured 

 and yellowish-gi'een lichens are especially rich in usnine, for instance, Lecidea o-eo- 

 graphica and Parmeha sarmentosa. Usnine acts a conspicuous part by its various 

 metamorphoses and combmations in the alterations of colour of many liclaens. In all 

 lichens however it is accompanied by yellow or green resins, which in common with it 

 partake of the property of becoming red by ammonia and exposure to the air ; this red 

 colouring however is destroyed by sulphuretted hydrogen. Usnine occurs in the thallus 

 as well as in the fruit-discs. The shields of the Cladonise contain near the fruit-bearinnr 

 vesicles quill-shaped cyhnth-ical cells, which are coloured pale red at the base, but 

 darker towards the apex by a colouring substance, which dissolves in ammonia and 

 potash with a wine-red, m sulphuric acid with a carmine-red colour ; the sulphatic solu- 

 tion is precipitated by water ; the alkaline solution is not decolorized by sulphui'etted 

 hydrogen. The nearly scai-let-red fruit-discs of the Cladonite become brown and 

 blackishArown with age. In fact, the fruit-discs of the lichens contaming usnine are 

 precisely similar in colovir to the thallus, or brown, reddish-brown and camiiine-red. 

 The sulphur-yellow lichens contain most usnic acid, and indeed in a free state ; the other 

 colours are probably produced by the action of the alkalies and eai'ths of the vegetable 

 salts in the lichens, the ammonia of the rain-water assisting the chemical action of the 

 usnic acid, which is otherwise insoluble in water. In this manner the green, red and 

 brown colours may originate. The silver-white Cladonia i-angiferina probably contains 

 the usnic acid in the state of an earthy salt. Lecidea geographica is sometimes 

 sulphur-yellow, sometimes yellowish-green. If some pure yellow specimens be sus- 

 pended in a glass over a solution of carbonate of ammonia, they become covered with 

 carmine-red globules, after frequent washing entirely lose the usnic acid, and finally 

 become grayish-white Uke dead lichens. The Parmelise and Usnese continue of a 

 brilliant green colour in shady and moist places, but when exposed to the heat of the 

 sun they become brownish-black ; if treated as above with ammonia and dried, they 

 likewise present similar colours. The fruit-discs of the Cladonioe also turn brown under 

 similar treatment. The cause of all these changes is the usnic acid, which itself is of a 

 yellow colour, but becomes o.xichzed in combmation with bases by exposure to the air, 

 forming various coloured compounds. Chem. Gaz. 1844. 182. 



A-^^J 



Fig. XXXIIl*. 



In this, as in the Fungal aUiance, I have forborne fonually to break up Lichens into 

 several Natui'al orders, and have preferred to leave the task to others moi'e skilled than 

 myself in this branch of Botany ; but it is not to be doubted, that hereafter the pro- 

 Mr. Berkeley, is not averse to this reduction. The Collemaceae have strictly the thallus of an Alga, and 

 the fruit of a Lichen. Tlie following genera are comprised in the group : — 

 C'ollema, Ach. I Sj-nalyssis, Fr. I Omphalidium, Met/, et I Lichina, Afi. 



Leptogium, /v. Myxopuntia, Mont. et\ Ftotw. \^lyT'\ans,mm, Mont, and 



I Dur. I Paulia, F^-e. \ Berk. 



It is better for the present, in a matter confessedly so difficult, to throw out the above in the form of 

 a hint, rather than to propose a distinct natural order. But every thing seems to indicate the necessi ty 

 of placing them apart under some kind of denomination. 



Fig. XXXIII*.— Parmelia tiliacea. 



