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 

 Mallotiurn (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- 

 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 hyphae and the Single FlG . I79 . Mallotium midebrandii, Garov. a radial longitudinal section through the 

 11 11 T) f fV, thallus; u the under side, b portion of a very thin section through the under side showing 



algal CellS. UUt tne SpC- tne rind, hairs, hyphae and Nostoc-filaments. a magn. 190, * 390 times. 



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, Fe'e, 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 



