Nov. 1S97.] BOTANICAL SOCIETY OF EDINBURGH 67 



species have been modified to suit the convenience of their 

 insect visitors, and how the colour had been acquired to 

 alhire them; but in such cases the object of the insect's 

 visit, so far as it was concerned, was entirely to satisfy its 

 own wants, yet, in so doing, it ministered unconsciously to 

 the welfare of the plant it visited. Instances of such 

 mutually advantageous connection between plants and 

 insects are nuinerous, but cases in which plants of widely 

 different order have been found to live in symbiosis with 

 each other are, so far as I am aware, of rare occurrence. 

 There are, however, two very notable instances of plant 

 symbiosis which I would take the opportunity of referring 

 to, though they are probably familiar to all who are here 

 present. 



In examining under the microscope the substance of 

 lichens, such as those to which old wayside walls owe 

 their beauty and colour, it is noticed that, while the moss 

 of the structure is composed of a fungus-like arrangement of 

 tissue, having a mycelium whose long hyphos form a vegeta- 

 tive network over the stone, penetrating its fine crevices, 

 and, while there are disposed on some part of the organism 

 the ordinary fungoid organs of reproduction, there are also 

 to be found, entangled or entrapped within the body of the 

 lichen, a number of little green-coloured cells, which in 

 botanical treatises were called gonidia, and whose object in 

 the economy of the lichen and connection therewith were 

 not understood. But for the presence of these green- 

 coloured cells, the plant would probably have been called a 

 fungus. It was not until a few years ago that Schwendener, 

 from a careful study of the matter, came to the conclusion 

 that what were described as lichens were not a distinct 

 class of plant at all, but really a copartnery of two quite 

 distinct organisms — a fungus and an alga. A fungus is a 

 plant which, however varied its form and habit may be, is 

 distinguished by one characteristic peculiarity, that it cannot 

 make its own organic matter, but can only vegetate upon 

 the organic matter made for it by other plants. On a bare 

 wall a lichen cannot find that organic matter, for no 

 vegetable organisms preceding it in time have left behind 

 them residues in the form of organic matter sufficient for 

 its needs. The only way in which a lichen can grow in 



