240 XIX. STRUCTURE OF THE THALLOPHYTA. 



fungus 'and alga, which is based upon reciprocal service. At the 

 under surface of the thallus of Anaptychia the fungal hypha& 

 again interlace more closely, so as to form a kind of under rind ; 

 or this closer combination does not exist, and the looser central 

 tissue extends to the ventral surface. This latter is in general 

 the case. At the edges of the thallus, however, the rind of 

 the dorsal surface, in all cases, extends underneath to the ventral 

 side. From these edge's arise, as we have already seen macro- 

 scopically, the fixing cilia (rhizines), which now can be made out 

 to consist of closely-combined parallel hyphae. The walls of 

 these hyphae have a brownish colour. At their base the threads 

 often fork. In other lichens the rhizines are apt to spring 

 mostly from the ventral surface of the thallus. Chlorzinc iodine 

 stains the walls of the gonidia immediately a beautiful blue, while 

 the hyphae only appear yellow to yellow-brown, the so-called 

 fungal cellulose. 



In Anaptychia ciliaris we have a lichen with what is called a 

 layered or heteromerous thallus, so called because the algal 

 zone forms a special layer in the thallus. In more lowly orga- 

 nised lichens the thallus is homoiomerous, i.e., the gonidia are 

 distributed through the whole tissue. To the last belong also 

 the gelatinous lichens, in which the gonidia lie in a translucent 

 jelly, which is traversed by the hyphae of the fungus. The Algae r 

 which take part in the formation of the lichen thallus, differ 

 according to their species, are coloured green or blue- green, 

 but belong almost: exclusively to the lowest divisions of the Algae, 



Alqa. Fucus vesiculosus. The thallus of the Algae shows a 

 wide range of structural variation, from very simple to relatively 

 complicated. As an example of the latter, we will take the olive- 

 green or brown sea-weed, Fucus vesiculosus, known as the " bladder- 

 wrack," and which is so common around our coasts. This Fucus 

 is, in its younger parts, flat, leaf-like, traversed by a midrib 

 projecting on both sides, and forks more or less regularly in the 

 plane of the leaf-like expansion. It usually bears bladdery swell- 

 ings, often situated in pairs, one on either side of the midrib, as 

 well as singly at the base of the bifurcations ; these bladders are 

 sometimes wanting. In older parts the leaf-like expansion of 

 the thallus is gradually disorganised, and finally is reduced to the 

 much thickened midrib, which has become elliptic in section, and 

 simulates a stem. The stem ends below with a rounded attaching 

 disk, or several stems may arise from a common base. From the 



