OBSERVATIONS ON COLLBMA. 25 



characters by Kiitzing, being considered by liim as somehow a por- 

 tion of the structure of the alga, which, indeed, itself reproduces 

 by conjugation, and is, no doubt, in i'act, a dcsmid. 



Bchwendenei claims as the foundation or basis for the production 

 of " Collemacese " only such nostochaceous plants as live in moist or 

 wet habitats — the entirely aquatic forms (Trichornras, SphaBrozyga, 

 Cylindrospermum, Dolichospermum), he considers, being inac 

 sible under water, are protected from the attack of the parasite, 

 and thus " cannot enter into the ' gonidia question.' " The fact 

 that these latter form independent "spore-cells" (reproducing the 

 plant), he would seem, so far as we can judge, to hold as having 

 no material, if any, bearing on the question, for he dwells only on 

 their being submerged as giving them an immunity. " But in any 

 case," he says, afterwards, further on, as regards the question, 

 " whether certain species of Cylindrospermum pass into the 'gonidia 

 state' [that is, become the basis of Collemacea 1 ] remains for so 

 long doubtful, till the transition, here alone decisive, be observed. 

 In the Collema-thallus itself a decision is of course no longer 

 possible, since the spores characteristic of Cylindrospermum ap- 

 parently just as little come to development in the gonidial state, as 

 do the ' manubria' of the Rivularieas." (This last allusion has a 

 bearing on Lichina, &c, which the author thinks have plants 

 appertaining to Rivulariea3 for their basis, but without manubria.) 

 I would venture to suggest were such Algre as these truly seized 

 upon by this completely innocuous parasite — nay, which, if tin' 

 hypothesis be true, rather tends to favour the growth and vigour of 

 the " gonidia" — we should hardly expect that, on the other hand, 

 the innate or inherited tendency to produce "spores" would at 

 the same time become wholly extinguished. It would, I should 

 venture to suppose, seem probable, even admitting the views of 

 Schwendener and Reess as regards Nostoc, that Cylindrospermum 

 is not likely to have anything to say to the "gonidia question."' 

 But the isolated observation, for the first time recorded in the pre- 

 ceding brief communication,* would seem to show that Nostoc, 

 too, may form spores, though it be, indeed, so very exceptionally, 

 and so extremely rarely. 



The main object, then, of the present communication is to offer 

 the following three suggestions which occur to me : — 



1. To suggest the possibility that, if we may conceive Dolicho- 

 spermum, &c, excluded from the "gonidia question" as forming 

 special fruit (that is, " spores "), so might we regard Nostoc as ex- 

 cluded, though its formation of spores be so extremely rare. 

 Seemingly, indeed, the formation of spores by an algal species, 

 supposed to become occasionally lichenized, is not a reason against 

 the hypothesis as viewed by Schwendener — he only assumes that 

 such an example of the alga surrenders, or leaves in abeyance, its 

 tendency to the production of spores. 



* " GreviPfca," Xo. 1. July, 1872, p. 10. 



