SCIENCE-GOSSIP. 



are taken to prevent too rapid plasmolysis. I have 

 some under the microscope by me now which have 

 been mounted about eight years. It is not easy to 

 see what advantage the Azolla gains by this 

 companionship, though there may be some ; but it 

 is certain the alga obtains protective security, and 

 may not improbably utilise some waste product 

 from its host. It is true very similar species, if not 

 identical ones, are plentiful enough without any 

 protecting plant. Their colour shows they are 

 able to fix the carbon dioxide found in the water 

 for themselves, and are thus fitted for an in- 

 dependent life. The Azolla is commonly grown 

 with other aquatic plants in tanks in warm green- 

 houses, and requires a higher temperature than our 

 open air in order to flourish, but it will live very 

 well floating on an aquarium at ordinary dwelling- 

 room heat during several months in the summer. 

 I have on one or two occasions obtained it from 

 botanic gardens, and no doubt anyone sufficiently 

 interested might get it that way on application to 

 the proper authority. I have also obtained it in 

 the summer from Mr. R. Green, Central Avenue, 

 Covent Garden Market, London, who supplies 

 various aquarium requisites, and probably would 

 be able to forward specimens by post if desired. 



Many cases are known where a Nostoc, or closely 

 related alga, takes advantage of cavities in other 

 plants to enter and dwell there; they may be found 

 in some of the Hepaticae and in the cells of 

 Sphagnum, also in the large empty cells forming 

 the velamen of the aerial roots of some epiphytic 

 orchids. As in these cases, it is probable the 

 tenant obtains no other advantage than that of a 

 comparatively secure resting-place, while the host 

 is not benefited by its presence, it might be more 

 appropriate to consider these as commensalism 

 only. 



The most perfect instance of symbiosis, however, 

 is one with which, from its commonness, all are 

 more or less acquainted, and which may be 

 investigated without difficulty by anyone possess- 

 ing a microscope. It is now generally accepted that 

 lichens are composed of two distinct organisms, 

 one an alga, the other a fungus. They live 

 together in companionship, each helping the other 

 to fight the battle of life. With their united 

 capacities they are able to occupy and flourish in 

 situations which neither could hold alone, places 

 in which no other plant could contrive to exist. 

 The alga is always one of the lower members of 

 the class, mostly unicellular, occasionally filamen- 

 tous. It has been found possible, in some instances, 

 to isolate the alga from its fungal companion and 

 to cultivate it as a normal independent plant. The 

 fungus is usually one of the Ascomycetes, though 

 occasionally another kind, but the attempt to grow 

 them without the alga has not succeeded. 



The satisfactory demonstration of lichen structure 



is not very easy, partly because of the small size of 

 the elements and partly because the fungus hypha? 

 are with difficulty wetted, so that even with a thin 

 section the tissues are apt to be filled with air, 

 which obscures the view. A fairly successful 

 method is as follows : trunks of trees, somewhat 

 damp walls, palings, etc., are often covered with a 

 layer of bright green ; if some of this is examined 

 with a one-fourth inch objective, it will be found 

 very commonly to consist of round cells, in some cases 

 single, in others in fours ; these are algae of the 

 Protococcus or Palmellaceae group. In places the 

 layer appears grey, instead of green, drier and 

 more powdery. If a specimen of this is examined 

 it will be found difficult to wet, and will appear 

 dark and ill-defined under the microscope owing to 

 entangled air. The grey colour and retained air is 

 due to the presence of fungus hyphae growing 

 around and between the cells of the alga, in the 

 denser parts matting all together, and forming, in 

 fact, one of the pulverulent lichens. Between the 

 patches of grey lichen and green alga portions may 

 be found in a transitory condition suitable for 

 examination, and in damp weather, especially, 

 observation of the composition of the lichens is 

 fairly easy. To attempt the examination of sections 

 cut from a Thalloid lichen in its natural state is 

 somewhat hopeless, as they seem to defy all the 

 usual methods of getting rid of air in such tissues. 

 I have had a Cladonia in weak spirit for months and 

 at the end of the time it was no wetter than at first ; 

 but pieces torn with needles and well soaked in 

 slightly warm water will often show the arrange- 

 ment of the algas, either in rows or scattered 

 irregularly through the thallus, according to the 

 species. It is believed that the fungus supplies to 

 the algae, water containing mineral matters in 

 solution, and receives in return carbo-hydrates, 

 which the algae is able to manufacture under the 

 influence of light, from a solution of carbon 

 dioxide, owing to the presence of chlorophyll in 

 its cells. It is probable that the amount of 

 reciprocity between the two elements varies 

 considerably among different species of lichens. 

 In not a few cases perhaps the fungus actually lives 

 as a parasite upon its imprisoned captive, giving 

 nothing in return ; but a more detailed consideration 

 of the physiology of the relationship would here 

 carry us too far and occupy too much space. 



In conclusion, we must carefully note that both 

 commensalism and symbiosis, though not parted by 

 any sharp boundary line themselves, are each 

 fundamentally distinct from parasitism. Further, 

 though it is true that organisms living in such close 

 relationship run a great risk of degenerating into 

 that condition, yet, when they do so, the amicable 

 companionship previously existing ceases at once, 

 and as parasites they have no place in our subject. 

 9, Agamemnon Road, West Hampitead. 



