202 LECTURE XII. 



that the chemical processes in nutrition, continually going on in the plant, and the 

 molecular movements during growth and the passage of ^uids from place to place, 

 are all connected with electrical disturbances of various kinds, although it has not 

 been possible hitherto to demonstrate this experimentally. We may also suppose 

 that in the ordinary life of land-plants especially, during the continually altering 

 differences of electrical tension between the atmosphere and the soil, equalisations 

 take place through the bodies of the plants themselves. The land-plant rooted 

 in the soil offers a large surface to the air by means of its branches, and the 

 roots are still more closely in contact with the moist earth, while the whole plant is 

 filled with fluids which conduct electricity and are decomposed by currents. Such 

 being the case, it can scarcely be otherwise than that the electrical tensions between 

 the atmosphere and the earth become equahsed through the plant itself. Whether 

 this acts favourably on the processes of vegetation, however, has not yet been 

 scientifically investigated, since what has been done here and there in the way of 

 experiments in this sense can scarcely lay claim to serious notice. 



We possess, on the other hand, more exact and deeper knowledge, reaching into 

 the very essence of the matter, as to the action of Chemical forces in the plant. Among 

 the very numerous chemical elements of the earth, there are but few which enter 

 from without into the body of the plant (either in the condition of an element, 

 as the oxygen of the air, or in the form of very simple compounds, as carbon dioxide 

 and water, or, finally, in the form of salts) and there undergo decompositions 

 producing new chemical combinations, from which the organisable substance of 

 the plant itself proceeds. It is the object of the theory of nutrition to study these 

 processes in detail. Mention is made of them here only in so far as they con- 

 stitute external conditions of the life of the plant. If only a single element of those 

 necessary to nutrition is by any chance absent, or is present in too small quantity, 

 a plant cannot be nourished at that place, and thus cannot even live for long. The 

 well-being of any plant, therefore, depends upon all the elements contributing to life 

 being present in suitable chemical combinations and available to the plant. That 

 nearly the whole surface of the earth is covered with vegetation, and that even the 

 waters and the sea abound in plants, results simply from the fact that the few materials 

 subserving life are almost universally present in the necessary combinations. Or, we 

 may say, plants are constructed from those chemical compounds which are present 

 in quantity almost everywhere on the surface of the globe. Apart from oxygen, 

 carbon dioxide and water, there are a small number of salts — potassium nitrate, potas-s 

 sium chloride, calcium and magnesium sulphates and phosphates, and compounds 

 of iron — which, as we know, suffice for the nutrition of every plant, and which are, 

 moreover, absolutely indispensable. These chemical compounds, however, are found 

 together nearly everywhere, though mingled in the most various relative quantities : 

 this influences the thriving of the plant for good or ill, and therefore co-operates in 

 determining its distribution. To mention only a few examples in this connection : 

 there are certain species of plants which flourish in fresh water only, and others which 

 do so only in sea water. A certain large number of land-plants grow only in the 

 neighbourhood of the sea, or around salt springs, or on the saline soil, of dried-up 

 seas in places abounding in common salt; which places, however, other plants 

 avoid as unsuited to them. The presence or absence of water at a given spot is 



