a primitive Lichen. 
583 
Multiplication. 
Both the alga and the fungus seem to be able to develop quite well 
apart, and multiplication of the Botrydina is probably due to this. Here 
and there among the thalli specimens may be seen in which the fungal part 
has a brownish tinge and appears to be disintegrating, while the alga 
appears to be growing actively. It seems likely in these cases that the 
fungal membrane finally ruptures and the alga is set free, though this has 
not been observed. In other cases it is the alga which dies. A specimen 
was seen in which the fungal tissue was quite intact, yet no green cells could 
be seen. From two adjacent cells of the fungal tissue hypha had been sent 
out with septa at short intervals (Fig. 9). The alga when set free divides 
and multiplies until the cells come in contact with the fungal hyphae, by 
which they are enveloped with subsequent formation of a new thallus. 
The Origin and Nature of Botrydina. 
The manner in which the cellular structure of the fungal tissue has 
arisen is not quite clear. From Fig. 4 it would appear that on coming 
into contact with an algal cell, a single hypha applies itself and swells 
up enormously round the algal cell until it has completely covered it, and 
then septa are formed. In Fig. 4 it will be seen that, though the algae 
have been completely invested by the fungus, no trace of septa appears. 
A somewhat later stage is shown in Fig. 5, where a few septa are present. 
When it has reached this stage connexion with the hyphae appears to 
be lost, for Figs. 10 and 11 show specimens only slightly larger in 
which there is no connexion. The envelope must then increase in size 
and form new septa independently of this hypha, while the alga continues 
to multiply within. 
The nature of Botrydina is fairly obvious from the foregoing description. 
It consists of an alga and a fungus growing symbiotically , and is therefore 
a lichen. It is certainly a primitive type of lichen ; yet the union at an 
early stage of several smaller thalli shows a tendency towards the formation 
of a large thallus, though the diameter never exceeds 500 /x. It differs from 
the soredial stage of a typical lichen in the complete absence of any trace of 
hyphae in many adult specimens, and in the cellular nature of the envelope. 
The structure of the envelope and the inner pseudo-parenchymatous tissue 
formed from it shows a striking resemblance to the structure of the perithe- 
cial envelope of an Ascomycete such as Eurotium. 
The assumption that Botrydina vulgaris is a lichen at once explains 
the difficulty experienced earlier in the work in obtaining pure cultures. 
The fungus would naturally develop in the nutrient agar, while the Knop’s 
solution would favour the growth of the algal constituent; so that the 
