50 CONTIGUITY AND CHANGES OF PLANTS. 



the processes are analogous in the animal body. It is said that per- 

 sons living almost exclusively on potatoes pass unchanged granules of 

 starch which is not the case when gluten or flesh is taken in due pro- 

 portion with potatoes, as the starch has thereby been rendered capable 

 of assimilation. Thus potatoes alone are said to be incapable of af- 

 fording much strength; but mixed with other substances they are 

 wholesome and nutritious. Hence a superabundance of food, such as 

 carbon, without a due proportion of nitrogen, cannot be converted by 

 plants into gluten, wood, albumen, or other parts of an organ, but is 

 excreted as sugar, wax, starch, oil, resin, &c. When supplied with 

 an excess of nitrogen, the quantity of gluten, albumen and mucilage 

 will increase, and ammoniacal salts will remain in the sap, as in the 

 beet when the leaves are taken off. 



Plants growing beside each other are mutually prejudicial to their 

 growth, if each requires the same kind of food ; otherwise they may 

 thrive, if in the same state of development. Annual plants are less 

 perfect and abundant by cultivation on the same soil for successive 

 years. Some plants improve the soil while others, and the most nu- 

 merous, impoverish it ; some slowly and others speedily. All this, 

 however, depends on the supply by art of those materials which are 

 the peculiar wants of plants ; and this, not by an indiscriminate ap- 

 plication of nutriment, or manure, but such as affords in required 

 proportions the component parts of their organs. Their roots, doubt- 

 less, imbibe many substances not adapted to nutrition ; but these are 

 expelled into the soil as excrements from other parts of the roots. 

 Hence the more of them thus expelled the less fitted is the soil for the 

 growth of plants of the same species. Still, other kinds of plants 

 may require for their growth the same matters which have been ex- 

 pelled, and thus the soil may be made fertile for dissimilar crops. 

 These, again, expel substances which were nutritive to the previous 

 plant and a repetition of such, after a few years, may therefore be ad- 

 vantageous. 



The change of vegetable productions appears evident in nature from 

 the well known fact that forests of oak, maple, etc., when destroyed 

 or worn out" or disappear as they often do in our country from par- 

 ticular soils, are spontaneously followed by trees of the fir tribe; 

 and again, when pine forests are cut down, young oaks or other 

 similar species will shoot up in their places. Excretions from the 

 roots are more abundant in the night time than during the day. 

 Some plants if placed in water impregnated with the excrements of 

 others are impeded in their growth and prematurely fade, while others 

 grow vigorously. The matters thus expelled from the roots are acids, 

 gums, resins, etc., some of which are poisonous and others nutritious 

 to particular plants. Analogous circumstances are very obvious in 

 the animal kingdom. It is worthy of remark, however, that a change 



