HICKS_, ON GONIDIA OF LICHENS. 93 



(fig. 13 b). The quarternary forms liave a resemblance to the 

 "tetraspores " of the algae^ and may possibly be homologous 

 with them. It would be worth while extending this observa- 

 tion to Lichina. 



Having thus shown various lines of development through 

 which the Collema- and Nostoc-gonidia pass into Nostoc, I 

 shall now bring forward instances in which it will be seen 

 that Nostoc, by producing the colourless fibres and the so- 

 called epidermic layer^ tends to revert to its parent Collema. 



In old masses of Nostoc, especially where they have been 

 removed to a dry situation, it will not be difficult, by careful 

 search, to find within its substance portions, here and there, 

 in which fibres are developed possessing every character of 

 those in Collema. I have represented this condition at fig. 

 11, which was taken from a large mass of Nostoc. The pre- 

 cise origin of the fibres I did not make out — whether from 

 the vesicular cells or not ; but they were unmistakeable in 

 their appearance. Again, in Nostoc derived from those Col- 

 lemas which have a so-called epidermis, I have found them, 

 by keeping in dry situations, to have a tendency to produce a 

 similar layer — CAddently from changes in the vesicular cells 

 in the mode represented at fig. 12. The various vesicular 

 cells in a neighbourhood become enlarged, lobed, or branching, 

 and jointed; when these portions come into contact, and so pro- 

 duce the appearance of a reticulated epidermis of the Collema. 



If, in addition to this, we refer to the remarks I made at 

 first, that the Nostoc balls which were extruded from the 

 thallus of Collema (figs. 5, 6) had so strong a disposition to 

 throw out these fibres, that very soon they passed into Col- 

 lema (fig. 7), the connection is thence apparent; for retard 

 the fibre-growth at the same time that the gonidial growth 

 proceeds, we then shall have a mass of Nostoc. 



Thus a connection is established in both directions between 

 Nostoc and Collema. 



But, as I have before alluded to, the power of producing 

 Nostoc is not confined to Collema. 



One instance I have met with, in Avhich the gonidia of a 

 gymnocarpous lichen, whose apothecia, with theca and spores, 

 are figured at fig. 15, a, b, developed themselves into Nostoc 

 balls while still beneath the apothecia, and within the thallus, 

 which was crustaceous. 



Fig. 14 represents a section of an immature apothecium, 

 the gonidia beneath being unchanged. Fig. 16 shows a por- 

 tion of the thallus, in which one gonidium is undergoing 

 the Gleocapsoid change, as at figs. 1 b and 8 a. The further 

 changes which the gonidia pass through, in order to arrive 



