208 BOTANY pakt i 



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geological formations ; silicon is indispensable in the case of the Diatomaceae (^^). 

 Aluminium, although like silica everywhere present in the soil, is only in excep- 

 tional instances taken up by plants (Lycopodiaceae, Lichens, Vitaceae, Legu- 

 minosae). Some species of Lycopodium contain a sufficient quantity of acetate 

 of aluminium to render the sap useful as a mordant. The same salt is found 

 also in Grapes. Aluminium is stored uj) in masses in the cells of the leaves of 

 American species of Symplocos (-^). On the other hand, although scarcely a trace 

 of iodine can be detected by an analysis of sea-water, it is found, nevertheless, 

 in large quantities in seaweeds, so much so tliat at one time they formed the 

 principal source of our supplies of this substance. 



The substances which, as culture experiments show, are not indispensable for 

 the life of the plant are, however, of use in so far as they can replace for some 

 purposes (such as the neutralisation of free acids, etc.) essential elements of plant 

 food. The latter are thus available for the special purposes for which they are 

 indispensable. Thus K can be partially replaced by Na, and Mg by Ca. 



Certain other substances, althongh not indispensable, ai-e of use in the plant 

 economy and of advantage to growth. Thus, for example. Buckwheat flourishes 

 better when supplied with a chloride, and the presence of silica is advantageous 

 as contributing to the rigidity of the tissues. It has also been discovered that by the 

 presence of certain substances, in themselves of no nutritive value, the absorption 

 of actual nutritive matter is increased. In minimal doses poisonous substances 

 often have a favourable eifect ; they lead to better utilisation of the substances at 

 the disposal of the plant. Their effect is, however, often injurious even when 

 diluted so as to be imperceptible to chemical tests ; thus by such " oligo-dynamic " 

 influence, copper sulphate, even in the proportions of 1:25,000,000, has a fatal eff"ect 

 on Spirogyra, and on Peas in a water-culture (^*). 



Under natural conditions some plants absorb the nutrient salts 

 from water as do the plants in a water-culture experiment. This is 

 the case in many water plants in which the whole external surface is 

 of use in absorption. Since the salts only exist in very dilute solution 

 in the water, the need of an extended surface for this purpose is 

 readily understood ; this in part explains the frequent occurrence of 

 finely divided leaves in water plants. The salts dissolved in the water 

 are not, however, sufficient for all aquatic plants ; many absorb 

 substances from the soil underlying the water by means of their roots, 

 and do not succeed when deprived of roots (^^). 



As a rule in the higher plants the salts are absorbed from the soil. 

 The salts contained in the nutrient solution described above, or similar 

 compounds, are constantly present in the water of the soil ; some of 

 them, however, in such small amount as only to suffice for the growth 

 of plants for a short period. Other sources of supply of the food-salts 

 must exist when such growth continues. In fact, the amount of salts 

 dissolved in the soil-water is no measure of the fertility of the soil. 

 The soil always contains food-salts, partly in an al)sorbed condition, 

 and partly in mineral form which the plant has to render accessible. 

 This is ertectcd mainly l^y the excretion of carl)onic acid from the root- 

 hairs. Many substances are much more readily soluble in water con- 

 taining carbonic acid than in pure water. In spite of repeated asser- 



