106 Mr. J. Miers on the Ehretiaceae. 



2. Zoospores have been discovered in three 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 with chlorophyll. 



8i We have demonstrated the identity of free gonidia with 

 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, Cladonia, 

 and Evernia led us to expect that other lichens would afford 

 forms corresponding with rudimentary Algae. Our researches 

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

 of Peltigera and of Collema, 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 Peltigera and 

 Collema continued to live when separated from the fh alius : 

 those of Peltigera were identical with an Alga called Poly- 

 coccus ; those of Collema produced organisms similar to Nostoc. 

 Consequently these three genera of Algae, 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 Ehretiaceae. 

 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 plants within my reach, referred to Ehretia, 

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

 those species which 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, so.me 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, each obcuneiform branch 

 bearing two distinct sessile stigmata ; but the placentation of 

 the ovary is that of Ehretia and Phatdia. 



The greater number of the Neogean species of Ehretia enu- 



