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ginning in the same way and tending to distention, becomes the 

 muriform spore, which when the color is absent, as is frequently the 

 case, is called decolorate. Between these two is a well defined in- 

 termediate type, the polar-bilocular, in which the locules are situ- 

 ated at the two extremities of the spore. But there are apparent 

 exceptions to this rule, as in the Caliciums and Arthonias, in which 

 the spores, according to Minks, are formed on another plan ; and 

 some elongated spores show indications of color. The spores are 

 expelled from the thekes most readily when the plant is in its fresh 

 state, and it is therefore advisable to study a Lichen before it has 

 been long collected. They are not always to be found in every hy- 

 menium, and in some Lichens, as Arthonia, they disappear by ab- 

 sorption or in some other way, very soon, and only the empty Theke 

 is to be seen. Spring and early summer appear to be the most fa- 

 vorable seasons for collecting Arthonias. The absence of spores, es- 

 pecially in the lower Lichens, often renders it difficult to determine 

 the species ; and species with normally muriform spores often occur 

 with unripe spores only transversely divided. Species have doubt- 

 less been separated on such specimens, of plants which are really 

 one. The spore is often filled with granular contents. Sometimes 

 there are two or more globules, but spores containing these are not 

 to be mistaken for bi-quadrilocular spores. The addition of aqua 

 potassa has the effect to render the internal divisions of the spore 

 more distinct. 



Spores vary greatly in size, from extremely minute to compara- 

 tively very large cells. The smallest will measure hardly a thou- 

 sandth of a millimetre in diameter, while the largest may have a 

 length of two hundred and fifty thousandths, and a breadth of sixty 

 thousandths of a millimetre, or even more, and are easily visible under 

 a common lens. The measurement of the spores is a matter of 

 some importance, and this is not difficult. The microscope should 

 be provided with an eye-piece micrometer. Then let a stage micro- 

 meter, divided by thousands of a millimetre, be placed on the stage 

 of the instrument, and let the observer note how many of its divis- 

 ions are equal to one division of the eye-piece micrometer. He will 

 thus be enabled to construct a table of dimensions in thousandths of 

 a millimetre, which may be written on a card for reference. If he 

 has a small rule, say a decimetre in length, laid off to thousandths 

 of a millimetre, he can at any time sketch a spore to scale of 1000 



