CHAPTER VII—-PHENOMENA OF VEGETATION.—-LICHENS. 413 
Nostoc-colonies attacked by the Lichen-forming Ascomycete. The hyphae of the 
Fungus intrude between the cell-rows of the Alga, which are strung together like the 
beads of a rosary interrupted by heterocysts and imbedded in a firm jelly; they 
branch in a manner which varies according to the species, and determine the general 
growth of the compound thallus. ° 
In most forms numerous peripheral branches of the hyphae run vertically into 
the surface of the gelatinous colony and end blindly in it. Leptogium, Obryzum and 
Mallotium (Fig. 179) are exceptions to this, in which the extremities of the peripheral 
branches pass into a rind or outer membrane, a simple or in places a double layer of 
polyhedric tubular cells without interstices with pellucid contents and colourless or 
brown walls, which covers the whole thallus. The membrane of the cells is often 
stronger on the upper side of the thallus and thickened more on the outer than on the 
inner side, recalling the epidermis of the higher plants. Numerous multicellular hairs 
spring from all parts of 
the cells of the lower 
rind in Mallotium, some- fg 
times short and isolated, 
sometimes longer and 
united in bundles; they 
serve to fix the thallus 
to the substratum. 
No direct and in- 
timate connection can be 
perceived in most species 
between the branches of 



the hyphaeand the single ric. 179. Madlotivm Hildebrandit, Garov. a radial longitudinal section through the 
algal cells. But the spe- the rit ham typhee and NonorAummme magn to Open 08 
cies of Plectopsora and 
Physma are distinguished by the presence of haustoria, short hyphal branches which 
grow in the direction of any Nostoc-cells and either lay their conical extremity close 
upon them (Arnoldia minutula, Born.), or penetrate into them (species of Physma). 
The cells thus attacked become at first larger and fuller of protoplasm than those 
which are untouched; but according to Bornet they die prematurely (see Fig. 167 C). 
Of these Nostoc-Lichens which have no rind the Gloeocapsa-lichens or Ompha- 
larieae (Omphalaria, Synalissa, Thyrea, Paulia, Fée, Peccania, Mass., Enchylium 
and Phylliscum, Nyl.) are really distinguished only by the Algae, which belong 
to the genus Gloeocapsa and its allies. Their cells are round and multiply by 
division in three directions, and after division they separate from one another inside 
the broad gelatinous membranes, which are stratified as a result of their mode of 
formation and hold the cells together in one mass. 
Of the branches of the hyphae some only thrust themselves into the gelatinous 
membranes, some attach themselves by their blunt extremity directly to the individual 
cells, so that most of the cells are borne on them as on a slender stalk. Each 
division of the algal cell is followed by a branching of the stalk, which thus supplies 
the new stalk that is wanted (Fig. 167 D, 180). Hence the cymose grouping of the 
stalked algal cells, which is a very striking feature in some species, and which 
