L ic hen-forming Fit ng i. 4 7 



striking advance has recently been made in our know- 

 ledo;e of tlie internal structure of other classes of 

 plants^ and especially of the Algte ... I need only 

 call attention to the discovery of sieve-tubes in the 

 larger brown Algse. The work of Parker, Will, and 

 Oliver has shown that these structures are in all 

 respects comparable to the sieve-tubes of the highest 

 plants — a surprising result, which by itself is sufficient 

 to show that the term * cellular- plants ' can no longer 

 be applied generally to the Algas. There can be no 

 doubt that further investigation will bring to light a 

 very high differentiation of tissues in some of these 

 plants. ""^ 



Concerning the primary divisions ofthe ThallopJiytes, 

 the Algds and the Fungi are yet considered as pos- 

 sessing their individuality. The Lichenes, however, 

 have been proved beyond doubtto consist of fungi para- 

 sitic upon alga3. It has been already stated that nu- 

 merous fungi are parasitic on phanerogams or flowering 

 plants, but in most such cases the fungus is parasitic 

 in the generally accepted sense of the term parasitic, 

 the benefit being all in favour of the fungus and to 

 the decided disadvantage of the host, whereas in 

 lichers — fungi parasitic on algce — both parasite and 

 host mutually benefit, and frequently to such an ex- 

 tent that neither parasite nor host can exist indepen- 

 dent, but to this statement there are proved excep- 

 tions. This condition of things is expressed by the 

 various terms, cominensalism, mutualism, symbiosis, 

 &c. The algal portion of the ftmgtis, possessing 

 chlorophyll, assimilates carbonic dioxide, and forms 

 organic carbon compoundsj while the mycelium of the 



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