RECENT OBSERVATIONS ON THE GONIDIA QUESTION. 119 



production of numerous branches ; the vigour of the develop- 

 ment evidently bears a relation to the mass of the alga. 



When the hypha penetrates into the frond it may become 

 equally distributed in the mass of the alga (Ephebe, Syna- 

 lissce), and then the general form of the alga is but little 

 modified. But more frequently the increase of the hypha- 

 threads takes place in a determined direction : when they are 

 parallel the fronds become cylindrical or clavate {Sytialissa 

 conferta), when they are radiate or fanlike they give rise 

 to orbicular {Omphalaria) or lobed fronds (Collema), in 

 the form of which the alga co-operates but feebly. But 

 in the great majority of lichens the hypha envelopes the 

 alga ; a more or less embracing network surrounds the 

 host-plant. It is not rare to see organs of fructification 

 appear on a thallus hardly more than beginning to be con- 

 stituted. 



The theory of parasitism, the author urges, explains the 

 origin of dead gonidia found in the deeper parts of lichens ; 

 it does away with the remarkable fact of the coincidence in 

 the same thallus of dissimilar gonidia, some containing 

 chlorophyll, others phycochrome — a very important distinctive 

 character between two great groups in the lower algae j it 

 explains, at the same time, the almost identity of the gonidia 

 of very diverse lichens, and the marked differences between 

 the gonidia of certain other lichens of which the thallus and 

 fructification are identical. 



To each single species or genus of lichen there does not 

 correspond a different alga : on the contrary, a small number 

 furnish the gonidia for a great variety of lichens. Some 

 lichens, under certain circumstances, accessorily invade algje 

 of a species different to those which normally form their 

 gonidia. But is there sometimes a complete substitution of 

 one species for another ? Pannaria triptophylla shows, in- 

 deed, that this substitution is possible to a certain extent, 

 although Bornet has in vain sought in very many lichens 

 to effect the substitution of Protococcus by Trentepohlia 

 (Chroolepus). 



Bornet concludes by admitting that, as regards the ordi- 

 nary mode of life of algse and lichens, there exists a certain 

 antagonism. Moisture, abundant and prolonged, which is 

 favorable to the algae, is injurious to the lichens ; plunging 

 the thallus of the latter in water for some time causes the 

 hyphse to perish. It was in this way that Famintzin and 

 Baranetsky set free the gonidia and obtained zoospores from 

 them. The partial death of the hyphse, due to indeterminate 

 causes, is sometimes encountered in nature. On examining 



