112 



POPULAR HISTORY OP LICHENS. 



therefore advise him, especially in his earlier microscopic 

 investigations, to accept the aid of Iodine and Sulphuric 

 acid as types of the two classes of reagents above mentioned. 

 To illustrate their action and uses it may be here advisable 

 to glance briefly at a few of the more prominent physico- 

 chemical characters of the Lichen- tissues and their con- 

 stituent elements. When applied to certain elements of 

 the reproductive tissues, the peculiar reaction of iodine with 

 starch is at once produced ; they immediately assume a rich 

 Prussian-blue colour : such is the case especially with the 

 theca. The iodine ought to be very diluted, and one drop 

 added on the glass slide to the tissue under examination 

 should suffice, for the very deep colour of the tincture of 

 iodine may obscure delicate shades of blue, and otherwise 

 interfere with the reaction or result produced. The theca is 

 most deeply coloured at its apex, where it is thickest. The 

 spore-sac and its contents, whether in the condition of a 

 grumous or granular protoplasm, or developed into spores, 

 are usually tinged orange or yellow ; and the theca, full of 

 mature sporidia, after the application of iodine, is thus a 

 very beautiful object under the microscope. The blue 

 colour of the theca is sometimes so deep, especially in its 

 young state, and the pale-yellow of its contents so obscured 



