106 Mr. J. Miers on the Eliretiaceffi. 



2. Zoospores have been discovered in tliree very different 

 genera of Lichens, viz. Physcia, Cladonia^ and Evernia ; and 

 as these genera were selected undesignedly, it is probable that 

 zoospores exist in all other lichens furnished Avitli chlorophyll. 



3. We have demonstrated the identity of free gonidia Avitli 

 the unicellular Alga Cystococcus of Nageli ; consequently this 

 is not a distinct genus, but only a phase of development of a 

 lichen. 



4. The culture of the freed gonidia of Physcia^ Cladom'a, 

 and Evernia led us to expect that other lichens would afford 

 forms corresponding with rudimentary Alga3. Our researches 

 prove this to be Avell founded. Vertical sections of the thalli 

 oi Peltigera and of GoUema, cultivated on moist earth, showed 

 the filaments in disintegration, the augmentation in size of the 

 gonidia, and their transformation into glomerules composed of 

 spherical cellules. The gonimic cellules of PeJtigera and 

 Collema continued to live when separated from the thallus : 

 those of Peltigera were identical Avith an Alga called Poly- 

 coccus • those of Collema produced organisms similar to Nostoc. 

 Consequently these three genera of Algre, hitherto regarded 

 as different and distinct, are in reality only the gonidia of 

 lichens in a state of development when separated from the 

 thalli which produced them. 



XVII. — On the Ehretiacere. 

 By John Miers, F.R.S., F.L.S., &c. 



Ehretia. 



This genus, as arranged by DeCandolle, is very heteroge- 

 neous, and requires redistribution, as it contains several dis- 

 tinct groups easily recognized by good characters, especially 

 by those founded on their carpical structure. After the exa- 

 mination of all the jdants within my reach, referred to Ehretia^ 

 from the New World, I propose to retain in the genus only 

 those species wdiich are proximate to E. tinifolia, Linn. Many 

 of those belonging to the Old World will probably be found, 

 upon critical examination, to be foreign to the genus. I have 

 not had leisure to analyse them ; but among those which I 

 have examined, some distinct forms have been noticed. A few 

 from Australia and Asia have a fruit containing four nucules, 

 each 2-celled and 2-seeded, with a particular organization ; 

 others, again, have a bifid style, cacli oljcuneiform Ijrnnch 

 bearing two distinct sessile stigmata ; but the placentation of 

 the ovary is that of Ehretia and Ehahdia. 



The greater number of the JSTeogean species of Ehretia cnu- 



