Oak Tree Trunks and their Vestments 491 



olden times it was a valued article of food, and is said to be 

 used in Egypt to this day to flavour bread. 



In our third group we have species very different from the 

 above. They form well defined dark green or whitish 

 patches 2 to 4 inches across, closely adherent to the bark. 

 The centre of the patch is occupied with wart-like bodies. 

 The commonest is Pevtusaria communis ; under a pocket lens 

 the warts are seen to be pitted with little depressions with 

 dark centres, the whole much resembling a sarsen stone. 

 The grey patch with the very conspicuous little heaps of 

 white powdery stuff upon them are probably only a peculiar 

 form of the P. communis. The condition is known as sorediate 

 (see p. 456). 



Another common representative of our group is the Leconora 

 subfusca. It may be at once recognised by the abundant 

 chestnut-brown circular warts scattered over a sharply defined 

 smooth greyish base. Under a lens each wart is seen to 

 have a greyish-white margin. A lady upon first seeing it 

 under a microscope said it reminded her of a lemon cheese- 

 cake. 



In our fourth and last group we have the writing lichens, 

 as we may term them. Their fructifying parts (apothecia) 

 simulate oriental writing. (One of the best known British 

 examples is Gf aphis elegans, common on holly bark). Here 

 is one, a little grey patch with black Hebrew-like characters 

 upon it. If you desire its name it is probably the Opegrapha 

 vulgata. Examine some of the "writing" with your lens. 

 An apothecium of this shape is termed lirellate by botanists, 

 a word derived from the Latin " lira," a ridge between two 

 furrows. You may judge for yourself if it is an appropriate 

 one. What may be the age of these lichens we cannot say. 

 How old is the oak ? Perhaps it has been growing for more 

 than a century, and the lichens, or some of them, may be 

 almost as old. Berkeley, in his " Introduction to Cryptoga- 

 mic Botany " writes, " Some of the large patches of Parmelia, 

 which occur on rocks, are of very great age. Patches of such 



