CHAPTER X. THE THALLOPHVTA. 33 I 



vegetative processes are not destroyed, but in some instances 

 even stimulated, for a time at least, so that over-production or 

 hypertrophy takes place, as when galls are formed by the stings 

 of insects. 



Some idea of the character of this association between para- 

 site and host may be gained by the study of Fig. 534 A and B, 

 taken from Bornet's researches on the Lichens. Here portions 

 of tissue from two different Lichens are shown. 



But while from the investigations of Schwendener, Bornet, 

 Stahl and others, there can no longer be any doubt as to the 

 composite character of the Lichenes, they nevertheless form a 

 group so distinct in appearance, that their real relationships to 

 Algae and Fungi were not even suspected until recently. 



They are very abundant plants, growing everywhere, on the 

 bark of trees, on fences, decaying logs, rocks, and on the ground. 

 Some form leaf-like expansions ; others closely encrust the sur- 

 face on which they grow, and others still are fruticose, resembling 

 small branching shrubs in appearance. 



Their colors vary from almost white to greenish-gray, yellow, 

 orange or brown. Examined microscopically, their tissues 

 always show colorless filaments, the fungus hyphae, and green or 

 red chlorophyll-bearing cells which belong to the Algae. The 

 latter are technically called gonidia. 



In some species it is the alga element which is the larger 

 factor in determining the shape and structure of the plant ; in 

 the majority, it is the fungus, and the alga is only a subordinate 

 factor. In some Lichens the algae are equally distributed 

 throughout the thallus ; in others they are arranged in definite 

 groups or layers. The former kinds are described as homoiom- 

 erons, the latter as hetero?nerous. All the species are capable of 

 enduring dessication without destroying their vitality. 



The fructification of Lichens is, as we have seen, that of the 

 fungus and not of the alga. In all those whose life-history has been 

 traced, there are male and female organs of reproduction. The 

 former are roundish or flask-shaped cavities or conceptacles in 

 the thallus called spermogonia, which, when ripe and wet with 

 water, emit very numerous minute spermatia from a narrow open- 

 ing. The archicarp, or ascogonium as it is also called, is at first 

 a thickish hyphal branch which becomes twisted into close coils 



