the Change of the Gonidia of Lichens into Zoospores. 103 



remained alive, considerably increased in size, and multiplied 

 by division. Some time after, minute punctiform bodies, of a 

 beautiful green colour, appeared amid the decomposing thallus, 

 which were gradually transformed into new thalli of Physcia 

 ciliaris. 



Our own researches on other lichens have fully confirmed 

 those of M. Spersclmeider in all particulars, except that we 

 have not succeeded in recognizing the transformation of the 

 free gonidia into new thalli. 



Our own experiments were as follows : — Many thin slices 

 of the thallus of Physcia parietina were placed on bits of bark 

 of fir and lime which had been previously boiled in water for 

 some minutes, and which were afterwards kept in a humid 

 atmosphere. These bits of bark were placed in small salvers 

 on the bottom of a large glass vase, into which we had poured 

 a little water, and covered with a pane of glass. They were 

 thus maintained in a moist state for many months. 



Although this method was attended with good results, we 

 preferred another, by which the gonidia were isolated. The 

 entire thallus was, for many weeks, either immersed in water 

 or kept continually moist by water dropping upon it. The 

 filaments of the thallus decomposed rapidly, but the gonidia 

 preserved their vitality. The entire mass was washed in pure 

 water, and deposited on bits of lime-bark. The gonidia iso- 

 lated by either of the above methods always exhibited the 

 same changes. 



Each gonimic cellule, whether enclosed in the thallus or 

 isolated, presented a large central nucleus, as well as a large 

 lateral vacuole. In this state it perfectly resembled a uni- 

 cellular Alga called Cystococcus , described by M. Nageli in 

 his work ' Les Algues unicellulaires,' and figured on tab. 3, E.e. 

 We succeeded in observing, at a later period, in these gonidia 

 all the other phases of development of Cystococcus described 

 by M. Nageli, and have thus established the identity of this 

 Alga with the isolated gonidia of Physcia. 



l)uring the first days the gonidia augmented their size, but 

 preserved their spherical form ; afterwards they underwent 

 the changes corresponding to the metamorphoses observed in 

 Cystococcus by M. Nageli. The most remarkable of these 

 changes consisted in the transformation of the contents of the 

 gonimic cellules into zoospores. A portion only of the gonidia 

 were thus changed, the rest became divided by partitions into 

 a great number of cellules, which gradually-bccame of a round 

 form, and ultimately disunited by separation. 



The formation of the zoospores is preceded by a change 

 characteristic of the contents of the cellules. The outlines of 



