INTRODUCTION xxvii 



they should be damped and then pressed before being laid away : the process 

 flattens them, but it saves them from the risk of being crushed and broken, 

 as when dry they are somewhat brittle. Moistening with water will large!}- 

 restore their original form. All parts of the lichen, both thallus and fruit, 

 can be examined with ease at any time as they do not sensibly alter in the 

 herbarium, though they lose to some extent their colouring : the blue-grey 

 forms, for instance, often become a uniform dingy brownish-grey. 



Microscopic examination in the determination of species is necessary in 

 many instances, but that disability — if it ranks as such — is shared by other 

 cryptogams, and may possibly be considered an inducement rather than a 

 deterrent to the study of lichens. For temporary examination of microscopic 

 preparations, the normal condition is best observed by mounting them in 

 water. If the plants are old and dry, the addition of a drop or two of potash 

 — or ammonia — solution is often helpful in clearing the membranes of the 

 cells and in restoring the shrivelled spores and paraphyses to their natural 

 forms and dimensions. 



If serial microtome sections are desired, more elaborate methods are 

 required. For this purpose Peirce' has recommended that " when dealing 

 with plants that are dry but still alive, the material should be thoroughly 

 wetted and kept moist for two days, then killed and fixed in a saturated 

 solution of corrosive sublimate in thirty-five per cent, alcohol." The solu- 

 tion should be used hot : the usual methods of dehydrating and embedding 

 in paraffin are then employed with extra precautions on account of the 

 extremely brittle nature of lichens. 



Another method that also gave good results has been proposed by 

 French^: "first the lichen is put into 95 per cent, alcohol for 24 hours, then 

 into thin celloidin and thick celloidin 24 hours each. After this the specimens 

 are embedded in thick celloidin which is hardened in 70 per cent, alcohol 

 for 24 hours and then cut." French advises staining with borax carmine : 

 it colours the fungal part pale carmine and the algal cells a greenish-red 

 shade. 



Modern research methods of work are generally described in full in the 

 publications that are discussed in the following chapters. The student is 

 referred to these original papers for information as to fixing, embedding, 

 staining, etc. 



Great use has been made of reagents in determining lichen species. 

 They are extremely helpful and often give the clinching decision when 

 morphological characters are obscure, especially if the plant has been much 

 altered by the environment. It must be borne in mind, however, that a 



1 Peirce i8q8. - French 1808. 



