Petrie. — On Vegetable Physiology. 439 



plant life ; one, too, which it costs the farmer much more to 

 supply in the form of manure than any other important ingre- 

 dient that cultivated soils are apt to be deficient in. 



The lesson which the farmer should learn from the new 

 knowledge is obvious enough. Every few years he must 

 break up his land and grow a crop of one or other of the com- 

 mon useful leguminous plants, such as peas, beans, lucerne, 

 or clover. His land will thus be sufficiently supplied with 

 nitrogenous compounds, and he will have to add to it only 

 such other less expensive and scarce elements of plant food as 

 the soil may require. Clovers, too, will be grown along with 

 grass-crops, but this method is not so likely to yield satisfac- 

 tory results, as clover-plants die out in a few years, and are 

 apt to induce the condition known as clover sickness. This 

 undesirable condition of the soil can be partly avoided by pro- 

 viding it with sufficient stores of potash and lime, and more 

 effectually, and also more economically, by following a sub- 

 rotation of clover-plants in the general rotation of crops — red- 

 clover, white-clover, and alsike being sown in alternation, 

 when the leguminous crop of the general rotation becomes 

 due. It is, however, clear that lands laid down in permanent 

 pasture cannot be greatly enriched by gains of nitrogenous 

 compounds accumulated by the roots of leguminous plants, 

 since hardly any useful plants of this kind can be grown in the 

 same soil for a long series of seasons. It is therefore only in 

 lands that are cultivated with some regularity that the farmer 

 can draw to the best advantage on the bounty of nature for 

 the enrichment of his land. 



I need not dwell upon the application of the new knowledge 

 to the economic management of forests, orchards, and gardens. 

 It must suffice to point out that it has obvious and important 

 bearings on each of these pursuits. 



There are many other points connected with the relation 

 of the nitrogen of the air to the soil to which it would be in- 

 teresting to refer, but the time at my disposal will not allow 

 me to touch on more than one of them. 



Observers, even thirty years ago, had no doubt that the soil 

 in which plants grow was a comparatively simple medium, 

 consisting of a mixture of sand, clay, lime, humus, and 

 traces of other mineral substances. The modern botanist has 

 learned that the soil is a vastly more complex substance than 

 that. He has learned to recognise that every fertile soil 

 abounds in microscopic organisms, often to a degree that is 

 truly startling. The earlier estimates may be passed over as 

 less trustworthy than more recent ones. The latter, however, 

 are sufficiently startling, for tbey show that in a cubic centi- 

 metre of rich sandy soil from ten to forty millions of germs 

 may be found. In fact, we may look for from one to ten 



