36 THE PHYSIOLOGY OF PLANTS 



of cereals, which was formerly attributed to a deficiencjr of 

 silica in the lower joints or nodes of the culm, is known 

 now to be due to the incomplete thickening of the cell- 

 walls, in consequence of deficient illumination of the base 

 of the stalk. 



Of the substances which are occasionally present in plants, 

 and which are due to the habitat of the plants, iodine and 

 bromine must be mentioned as always present in marine plants. 

 The sea-water contains only very small quantities of these sub- 

 stances, while more than i per cent, of the dry weight of some 

 sea-weeds consists of these substances (poisonous for other 

 plants) combined with potassium. Some chemists have found 

 both these substances in bog and also in land plants, while 

 others have not been able to trace them in other specimens of 

 the same species. This shows how dependent the appearance 

 of these substances is upon the nature of the locality in which 

 the plants grow. 



Boron has been observed in traces in the Grass-wrack 

 (Zostera) and in some sea-weeds, while fluorine has been found 

 in the Club-moss {Lycopodium clavatum), in the Horse-tail 

 {Equisetum), and in grasses. 



Of the lighter metals which are usually found associated 

 with potassium, lithium and rubidium (not caesium) have been 

 found in some plants, the former, for example, in the tobacco- 

 plant and in thistles ; the latter in turnips and in the leaves 

 of tea and coffee. But in spite of their affinity to potas- 

 slum, they can in no way replace the same, but are, in fact,, 

 poisonous if they occur in any appreciable quantity. 



Allied to calcium is barium, which has been recognised in 

 the ash of several trees and shrubs, and strontium, which has 

 been discovered in several sea-weeds. The former of these twa 

 substances is harmless to many plants in fairly large quantities ; 

 the latter acts as a poison even in very small quantities. In 

 spite of the wide distribution of clay in soil and in rocks, its 

 chief constituent, aluminium, is confined in its occurrence to 

 very few plants (lichens and club-moss). This element occurs 

 both in rocks and also in the product of their breaking upj in 

 the soil, almost exclusively in the insoluble form of a double 

 silicate of potassium and aluminium. 



