302 SCIENCE AND FRUIT GROWING 



material, which had to undergo an elaborate manufacturing 

 process in the plant before it could be correctly described as 

 food. The soil was regarded merely as the storehouse of this raw 

 material, and the only recognised effect of the plant on the soil 

 was an exhaustion of the material which it contained. 



Running alongside of these ideas were others relating to the 

 action of one plant on another. It has always been thought that 

 certain plants had the remarkable power of injuring the crop 

 that came after them. In the seventeenth century the palmy 

 days of the Traveller's Tale Sir Hans Sloane gravely presented 

 the learned world with an account of a vegetable called the 

 Tartarian Lamb, which possessed something like four feet and a 

 body covered with a kind of down, and was remarkable among 

 plants in that it would suffer no vegetable to grow within a 

 certain distance. A later traveller searched for it in 1715, but 

 failed to find it in his journey from St. Petersburg to Ispahan. 



Traditions of very similar kind are still current, and it is main- 

 tained to this day by some who have been in the East that there 

 are plants growing which have a decidedly harmful effect on 

 vegetation. Most practical men will maintain that land which 

 has grown the same crop for a number of years becomes " sick," 

 and will not grow the same crop again, although it will grow a 

 good many others. It wants, in fact, a change. Among many 

 gardeners in this country it is not uncommon to speak of " onion- 

 sick land," while, on the Continent, " sugar-beet sick land " is 

 recognised by the growers. When nurserymen in this country 

 speak of " cucumber-sick soil " and " tomato-sick soil," in each 

 case it is presumed that the crop has done something to injure 

 the soil, although the injury is not necessarily felt by another and 

 totally distinct crop (see p. 336). 



These ideas appealed very much to the older botanists, and 

 were expressed clearly by a famous physiologist, De Candolle, 

 in a book published early last century, and which for many years 

 played an important part in scientific work. De Candolle 

 supposed that plants not only draw part of their food from the 

 soil, but also excrete into it certain waste substances, just as an 

 animal would, these excretions being harmful to other plants of 

 the same kind, but not to plants of a different order. 



Such an hypothesis readily explained why farmers prefer to 

 grow their crops in rotation, instead of continuously, and why 

 also certain crops, such as clover, will not grow year after year on 

 the same land. When the hypothesis was considered, however, 

 it was seen to be inconsistent with many obvious facts of nature : 



