172 BOTANY 



Next to them in quantity is nitrogen, which is derived principally from 

 the protoplasm. After combustion of the dried substance of plants 

 there always remains an incombustible residue, the ash, consisting of 

 the mineral substances contained in the plant. As these mineral 

 substances undergo transformation during the process of combustion, 

 they are found in the ash in different chemical combinations than in 

 living plants. From numerous analyses made of the ash of a great 

 variety of plants, it has been determined that nearly all the elements, 

 even the less frequent, are present in plants. 



In addition to the four already named, the elements found in the ash of plants 

 are sulphur, phosphorus, chlorine, iodine, bromine, fluorine, selenium, tellurium, 

 arsenic, antimony, silicon, tin, titanium, boron, potassium, sodium, lithium, 

 rubidium, calcium, strontium, barium, magnesium, zinc, copper, silver, mercury, 

 lead, aluminium, thallium, chromium, manganese, iron, cobalt, and nickel. 



Many of these elements, indeed, occur only occasionally and acci- 

 dentally, while others — sulphur, phosphorus, chlorine, silicon, potassium, 

 sodium, calcium, magnesium, and iron — are met with in almost every 

 ash. As might be inferred from the irregular occurrence of many of 

 the elements, they are not all necessary for nutrition, and although 

 their occasional presence in a plant may sometimes change certain of 

 its special characteristics (thus the presence of zinc produces the so- 

 called calamine varieties, such as, for example, Thlaspi alpestre var. 

 calaminarf, Viola lutea var. calaminaria, etc.), they do not exercise a 

 decisive influence upon its existence. 



The Essential Constituents of Plant Food. — Chemical analysis, 

 while enabling us to determine the substances present in plants, does 

 not show how far they are essential for nutrition. From culture 

 experiments, in which the plants are grown in a medium of which the 

 constituents are known, and kept under chemical control, it has been 

 ascertained that, in addition to carbon, hydrogen, oxygen, and 

 nitrogen, which form the principal part of the combustible elements 

 of the dry substance of plants, sulphur, phosphorus, potassium, 

 calcium, magnesium, and iron are absolutely indispensable to the 

 growth of all green plants. In the absence of even a single one of 

 these elements no normal development is possible. 



According to Molisch, only nine of these elements are required by the Fungi. It 

 is not, however, iron, as might be supposed, but calcium, that is unessential. On 

 the other hand, the ten substances named suffice for the nutrition of most green 

 plants ; but it is not to be denied that certain other substances are of use in the 

 plant economy and of advantage to growth, although not indispensable. 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 them- 

 selves of no nutritive value, the absorption of actual nutritive matter is increased 

 (cf. p. 175). In the case even of the very poisonous copper salts, experience 



