BRITISH LICHENS I HOW TO STUDY THEM. 243 



medullary layer of the thallus, for which purpose a small portion of the 

 cortical layer should be carefully removed by scraping it with a knife. 

 The results should be recorded on the paper bearing the specimens by the 

 aid of the following symbols. 



The hydrate of potash may produce a yellow colour, the symbol for 

 which is K + ; or it may only be a faint yellow, in which case it should 

 be written K f + . If the yellow tint changes almost immediately into 

 red, the symbol is K yellow then red. Should the yellow produced by 

 the hydrate of potash change into a deeper yellow when the hypochlorate 

 of lime is added it will be denoted thus— K yellow, C deep yellow. 

 Sometimes, however, the addition of hypochlorate of lime produces a red 

 colour, which must be written thus— K yellow, C red. Should the 

 hydrate of potash produce no colour, but, Jby the addition of the 

 hypochlorate of lime, a red colour is produced, the symbol is K — C + red ; 

 but if a yellow is produced it is K — C + yellow. Should neither re-agent 

 produce a reaction it is indicated thus — K — C — . Let us suppose, 

 however, that a reaction takes place in the medullary layer, but not in 

 the cortical layer, it will be then shown thus — K + , that is, the effect on 

 the cortical layer, which in this case is negative, is indicated by the 

 upper sign ; and that on the medullary layer, which is positive, by the 

 lower sign. Every possible chemical reaction given with these two 

 re-agents may be thus expressed in a brief and convenient form, facility 

 in the use of which can be acquired in a very short time. 



The next important step is to ascertain the microscopical characters 

 of the apothecium. The number, size, shape, and internal structure of 

 the spores ; the presence or absence of paraphyses ; the colour of the 

 hypothecium, if it be a Lecidea, together with any striking feature that 

 can be detected, should be carefully noted. Diagrams of the spores and 

 their micrometrical dimensions are of great value, and if made on the 

 paper bearing the specimens, it obviates the necessity of repeating the 

 work when subsequently referring to the specimen, as will often have to 

 be done in the course of future investigations. An excellent plan 

 pursued by some botanists is to prepare a microscopic slide of each 

 specimen, to be placed in a cabinet of convenient form for holding a 

 series, arranged in the same order as the specimens in the herbarium. 

 Such a collection will save an infinitude of trouble, for, although a 

 diagram of spores, &c, will be of great use, the inspection of a well- 

 prepared slide will satisfy the mind of a critical worker on points that 

 no diagram can adequately represent. When we say a well-prepared 

 slide, we do not mean to convey the idea that it should of necessity be 

 such a slide as would satisfy the fastidious taste of a dilettante micro- 

 scopist, though there is no objection to such slides, but we mean that it 

 should clearly display the essential parts of the object under examina- 

 tion, a quality that may co-exist with a certain rudeness of finish in 

 varnish and ornament. We will here lay before the student a few 

 practical directions for preparing his slides, as in so doing we shall be 

 able to convey incidentally some information on other practical points 

 in examining specimens. 



