262 BOTANY PART i 



Phosphoric aeid is connected with sulphuric acid in so far as it 

 is employed in the construction of at least some proteid substances, 

 especially the nucleo-protein of the cell nuclei; it forms from 0'3 to 3 

 per cent of this. In entering into the molecule of this substance the 

 phosphoric acid, unlike sulphuric acid, is not reduced. Lecithin (cf. 

 p. 258), which is present in all plants, also contains phosphorus, and 

 this is also the case for phytin, which occurs especially in seeds. 



The Metals. As may -be shown by the method of water culture, 

 potassium, calcium, magnesium, and iron are just as essential as any 

 of the substances hitherto mentioned. It is very probable, at least 

 for potassium and magnesium, that they take part in the construction 

 of certain compounds that are essential for the existence of the plant. 

 Probably protoplasm contains these elements. Other substances also 

 may contain them ; thus, for instance, a considerable amount of 

 magnesium has been shown to exist in chlorophyll. It was formerly 

 believed that chlorophyll contained iron because the chloroplasts 

 remained yellow when iron was omitted from the food solution. It 

 is now known that chlorophyll does riot contain iron and that iron is 

 also necessary for plants that are not green. This supports the assump- 

 tion that protoplasm itself contains iron, and that the " chlorosis " 

 which occurs when iron is wanting is a result of a diseased condition 

 of the protoplasm. 



Since potassium, magnesium, and iron thus pass into the substance 

 of the plant they must be assimilated, but we know nothing of how 

 or where this happens. The case of calcium is somewhat different ; 

 it is not invariably essential, for some Algae can succeed without it. 

 In other plants it has a protective function, preventing the poisonous 

 effects which result from iron, magnesium, potassium, and sodium, and 

 also from phosphoric acid, sulphuric acid, nitric acid, and hydrochloric 

 acid. It is, however, improbable that the indispensability of calcium 

 in the case of the higher plants is merely due to this protective 

 function. 



In speaking of insectivorous plants and of certain green parasites it 

 was mentioned that they might perhaps obtain their mineral food- 

 materials in organic compounds ; nothing certain is known on this 

 point. 



Water. We know that water is essential to the plant. When it 

 is taken into the plant as water without undergoing chemical change 

 we do not speak of its " assimilation." This is the case, for example, 

 in the water which fills the vacuoles of cells or that which permeates 

 the protoplasm and cell wall. It is different where the water is 

 chemically combined. This necessarily takes place when carbohydrates 

 are formed from carbon dioxide, and probably in other cases also. 

 In these cases there is the same justification for speaking of the 

 assimilation of the water as of the assimilation of carbon dioxide. 



