17 



little aqua potassa. The medullary layer of lichens is also often 

 colored blue by iodine. In most Placodiums the addition of aqua 

 potassa gives a beautiful violet reaction ; this reagent also often gives 

 a yellow or red color to the su)-faee or the medullary layer of the 

 Thallus of many Lichens. The variations of color produced by both 

 reagents have been much used by many Lichenists, especially by 

 Nylander, as the basis of specific differences. Nylander has cre- 

 ated a great many species based solely on these chemical reactions. 

 But the conclusion of Tuckerman, after an examination, was that 

 while they may be of instructive application to imperfect fragments 

 of specimens, and sometimes afford clews to affinity where there is 

 little to direct, as a whole their value as criteria is doubtful, as Dr. 

 Th. M. Fries had maintained in the introduction to his Licheno- 

 graphia Scandinavica. 



The spores vary in number in the Thekes from one to an indefi- 

 nite number. The most usual number is eight ; but it varies to a 

 limited extent in the same species or even in the same Apothecium. 

 By some authors the variation in number is made the basis of generic 

 or of specific distinctions ; by others, as distinctive only of varieties 

 or subdivisions of genera. In form they are globose, ovoid, ellipti- 

 cal, oblong, cylindrical, spindle-shaped (fusiform), finger-shaped 

 (dactyloid), club-shaped, or needle-shaped (acicular.) The ovoid 

 or elliptical is the most common form. In regard to color they are 

 either without color or of various shades of brown, now deepening 

 to almost black. Internally they are either simple, without any in- 

 ternal divisions or dissepiments, or divided transversely into two, 

 four or more, or numerous partitions, and are called respectively 2, 

 4, 6, 8, &c., locular and finally pluri, or multilocular, according to 

 the number of the separate divisions ; or by those who look at the 

 dissepiments, 2, 3, 4 septate, &c. When the transverse divisions 

 are again divided longitudinally the spores are called wall-shaped 

 (muriform.) The number of the loculse may vary in the same plant, 

 and in those with mqriform spores, the spore may be seen in all stages 

 of development. In its early stages the muriform spore is simple 

 or only transversely divided. Colorless spores may often be seen 

 associated with colored ones. Professor Tuckerman has distin- 

 guished between the typically colorless and the typically colored 

 spore. The former, commencing with the simple spore and tending 

 to elongation, becomes at length the acicular spore ; the latter, be- 



