AGARICUS LICHENS. 239 



refractive dot (cf. the figure). The section has here grazed a 



cross-wall, and the middle point shows the position of a pit, 



which is clothed, on either side of the partition 



wall, with a small aggregation of a highly re- 



fractive substance. The cells of the hyphae 



contain in the peripheral protoplasm, numerous 



very small nuclei. 



In the membranes of most Fungi Chitin has 

 been identified. For its identification the fungus 

 is cut into pieces ; these are first treated with 



FIG. 94. Agancus 

 dilute potash, then w r ith boiling dilute sulphuric oampestris. Part of a 



acid, afterwards with alcohol, and finally with 



ether. The white residue, w f hich upon drying hyphae the section has 



J grazed the cross- walls; 

 becomes hard and horny, possesses all the pro- a central point can be 



perties of chitin. It is insoluble in all reagents ^ up011 them 

 excepting concentrated acids. 



Lichens Parmelia ciliaris. We shall best obtain informa- 

 tion about the structure of the thallus of Lichens by means of 

 Anaptychia (Parmelia) ciliaris, very widely distributed on tree 

 stems ; and will confine ourselves in this place to the thallus 

 itself, and not take into consideration the fructifications (apothecia) 

 found upon it. The thallus itself is erect, leaf-like, and shrubby 

 (" foliaceous-fruticose ") ; on the dorsal (upper) surface, grey- 

 green to bright green ; on the ventral (under) surface grey. 

 From the edges of the thallus arise stiff cilia, which often be- 

 come lobed at their ends, and, where they extend to the sub- 

 stratum, adhere to it. We hold a piece of the thallus between 

 two pieces of elder-pith, and cut cross -sections through it. With 

 a sufficiently strong magnification, w r e see that the thallus consists, 

 on its dorsal surface, of closely interwoven, thick-walled hyphae. 

 These form the so-called rind, or cortical layer. Passing farther 

 inwards, the curves of the hyphae separate from one another, in 

 order to form the looser central layer. We can here readily make 

 out that the hyphae are long sacs, branched from time to time, 

 and divided by cross- walls. At the limits of rind and central 

 tissue lie scattered comparatively large, green, globular cells, the 

 Gonidia, They correspond with the Alga, Cystococcus humicola 

 ( = Chlorococcum humicolum). The hyphae fit closely to the green 

 algal cells, and carry to them crude nutrient sap, for which they 

 receive in return a portion of the substances assimilated in the 

 gonidia. There exists here a symbiosis, a conjoint existence of 



