LICHENS 



649 



general account of their curious organisation will here be attempted. 

 The algal portion of a lichen belongs to one or other of the lower 

 groups, and consists of cells termed gonids usually green, but 

 sometimes red or bluish-green interspersed among long cellular 

 filaments. The proportion between these two components of the 

 thallus varies in different examples of the type. Thus, in the 

 simplest wall-lichens the palmella-like parent cell gives origin, by 

 the ordinary process of cell-division, to a single layer of cells, which 

 spreads itself over the stony surface in a more or less circular form ; 

 and the ' thallus/ which increases in thickness by the formation of 

 new layers upon its free surface, has no very defined limit, and, in 

 consequence of the slight adhesion of its components, is said to be 

 ' pulverulent: But in the more complex forms of lichens the thallus 

 is mainly composed of long hypha*, which dip down into the superficial 

 layers of the bark of the trees on which they grew, and form by their 



FIG. 485. Lcpto<ii inn scotinum : Vertical section of the gelatinous thallus, magnified 

 550 times. An epidermal layer clothes the inner tissue, which consists for the most 

 part of formless and colourless jelly, in which the coiled strings of gonids lie ; 

 single larger cells of the strings (the limiting cells) are of_a higher colour ; between 

 them run the slender hyphse. (From Goebel's ' Classification.'] 



interweaving a hard crustaceous ' thallus,' in which the gonids are 

 imbedded, sometimes irregularly, sometimes in definite layers, known 

 as the yonidial layer (fig. 485), covered by an envelope of interlacing 

 filaments. It is from this algal portion of the structure that the 

 soredes of lichens are formed, little projections of the surface, com- 

 posed of single or aggregate gonids, invested by liypha?. and falling, 

 when dry, into a powder, of which every particle is capable of 

 reproducing the plant from which it proceeded. 



The fructification of lichens, on the other hand, is the production 

 of their fungal overgrowths, which are nourished by the algal 

 vegetation. The lichen-forming fungi, in fact, live upon their algal 

 hosts, like the endophytic fungi (such as the ' blights ' of corn), 

 which infest the higher forms of vegetation, each of the former 

 choosing its own alga, just as the latter mostly attach themselves to 

 particular victims. The peculiarity in the parasitism of the lichen- 

 fungi lies in the fact that they are not attached to their host externally 

 at any one particular spot, and do not penetrate into its cells, but 



