19 



to 1 by making the length and breadth as ascertained by his table 

 on the paper. If the spores are too large to be conveniently sketched 

 to this scale, it can be reduced one half or more. It is customary 

 to observe spores of the least and of the greatest size in a species, 

 and to indicate the dimensions thus, §^|^-nfa ™i^- i ^^^ upper fig- 

 ures indicating the length, and the lower the breadth. Spores meas- 

 ured when swollen by water will have rather larger dimensions, es- 

 pecially in breadth, than if measured dry. But the above method 

 will give a suflBciently close approximation to the dimensions given in 

 the books ; and after some experience the observer can form a fair 

 idea of the size of a spore, unless of great size, without measuring it. 

 Spores will vary in size in the same hymenium, some being twice as 

 large as others. To what extent differences in the size of spores 

 in specimens of the same genus are to be taken as specific is uncer- 

 tain. There are apparently some species in which two forms of 

 spores exist, those in one being constantly twice as large as those 

 in the other. An averaged sized spore will measure from 15 to 18 

 thousandths of a millimetre in length, by 6 to 9 in breadth. Below 

 these dimensions the spores may be called small or minute ; above 

 them, large to very large. 



Other Organs. 



It remains to notice one or two minor organs of Lichens. We 

 have already mentioned the Spermogones, or minute globular bodies, 

 resembling the Pyrenocarpous apothecia, which are situated upon 

 the Thallus, or (especially in Cetraria) on its margin. Internally 

 they contain filaments, now simple or branched, now jointed (Arth- 

 rosterigmata) , called Sterigmas, to which are attached laterally 

 or terminably ovoid or oblong cells called Spermatia. The spermatia' 

 are ovoid or oblong, staff-shaped and straight, or needle-shaped and 

 bowed, the smaller ones now spindle-shaped, or fusiform-thickened 

 at one or both extremities. They occur in great numbers, for which 

 it is diflflcult to account, unless on the supposition that they are pro- 

 duced and thrown off in succession. They were once supposed to 

 be sexual reproductive organs, but there is no proof that this is the 

 case. They are regarded by Nylander as indicative of specific dis- 

 tinctions, according to their various forms. But spermatia of dif- 

 ferent forms occur within the same genus. According to Minks, 

 the oflSce of the sterigmas is the development of the fruit-hyphse, 



