114 CJiapters in Modern Botany chap. 



inous plants and cereals in calcined earth and with a 

 supply of water that has been boiled, they obtained plants 

 without tubercles on their roots ; by watering these with 

 water which has been in contact with the soil in which 

 tubercled roots have grown, they were able to infect the 

 plants, or they could also inoculate them directly. After 

 infection or inoculation the plants acquire a new vigour, 

 and they increase in nitrogenous substances to a greater 

 extent than can be accounted for by the nitrogenous salts 

 in the soil. To explain this it seems necessary to believe 

 that with the help of their partner bacteria the legumes and 

 cereals are able to utilise the free nitrogen of the air. If 

 so, they are able to do what, as we shall afterwards see 

 (chap, ix.), is regarded by most botanists as quite impos- 

 sible for other plants. 



Symbiosis. — Whatever be the final verdict in regard 

 to the alleged partnership between bacteria and their 

 leguminous or cereal hosts, there are other cases of 

 mutually helpful partnership in regard to which there is no 

 doubt. To this De Bary in 1879 applied the name 

 Symbiosis (literally, a living together). It is interesting to 

 notice the parallels among animals ; zoophytes clustered on 

 the backs of crabs are comparable to epiphytes ; among 

 animals as among plants there are many external and 

 internal parasites ; the well-known partnerships between 

 hermit-crabs and sea -anemones are examples of the 

 beginnings of a truly co-operative symbiosis. 



The most important case of symbiosis among plants, 

 indeed by far the most fully-developed case in nature, is 

 that of the lichens. These used of course to be regarded, 

 as they no doubt popularly are still, as definite individual 

 plants united by common characters into a well-marked 

 group or "natural order," which botanists were wont to 

 reckon along with algte and fungi, but no less distinct 



