262 Professor J. Beynolds Green [Feb. 9, 



way in the water by means of vibratile filaments or cilia. After a 

 time they come to rest, secrete for themselves a protecting membrane 

 or cell- wall, and then by repeated division form a long filament. 

 Each protoplast in this thread is exactly like all the others, both in 

 structure and in properties and powers. In the higher plants the 

 first protoplast of the new individual or colony is not motile, and 

 is enclosed in some kind of cavity or receptacle, which differs in 

 different groups of plants. Its behaviour is, however, similar to 

 that of the one we have examined, and by repeated division of itself 

 and of those to which it gives origin, the complex plant is constructed. 

 In the larger plants a great deal of subsequent change or differentia- 

 tion takes place, which can be traced directly to the division of 

 labour which the massive size of the organism necessitates. 



In nature this general principle of co-operation is found to be 

 more wide-spread than this. We find inter-relations of varying 

 degrees of complexity existing between distinct plants, which co- 

 operate to a greater or less extent with each other for their mutual 

 advantage. The relationship is not confined to any particular group 

 of plants, nor is any corresponding complexity of structure or organi- 

 sation necessary for the establishment of such an association. We 

 find examples of the co-operation of flowering plant with flowering 

 plant, of fungus with fungus or alga, and of flowering plant with 

 either of the latter. 



The intimacy of the relationship is not the same in all cases ; 

 indeed the beneficial character of the association varies very greatly. 

 Sometimes it is almost all on one side ; sometimes each constituent 

 organism benefits equally ; in many cases there is a preponderance 

 of advantage on one side or the other. 



In cases in which the advantages of the alliance are more or loss 

 reciprocal we apply the term symbiosis to the association. It is im- 

 portant to bear in mind this limitation, as there are many cases of 

 the close association of two plants in which the advantages reaped 

 by one are attended with disadvantage and often serious injury to 

 the other. This feature is characteristic of what is known as para- 

 sitism. 



There are also instances to be met with in which the close connec- 

 tion of the two plants is attended by almost negative results, neither 

 gaming much advantage from their association. Of this, an ex- 

 ample is afforded by a few cryptogamous plants, particularly Antho- 

 ceros and Azolla, and by certain phanerogams, e.g. Gunnera and 

 Cycas, in cavities in the tissues of which particular Algae take up 

 their residence. In these cases neither organism appears to benefit 

 by the association, except that the alga finds a shelter and a home, 

 protected from difficulties incident to a watery environment. There 

 is no establishment in these cases of any co-operation between the 

 organisms, no formation of anything resembling a conjoint plant. 

 It is usual to apply the term commcnsalism to such an association 

 as this. 



