78 METABOLISM 



they produced by the agency of the vegetative organs themselves ? ' The 

 answer given by SCHRADER (1800) to this question ran as follows : ' Plants 

 develop these ash constituents by their own vital force.' Almost forty years 

 later (1838), the Academy of Gottingen again formulated almost identically the 

 same question : ' Whether the so-called inorganic elements which are found in 

 the ash of plants are still to be found there when they are not supplied to the 

 plant from without ? ' Such a question was certainly somewhat out of date. 

 DECANDOLLE'S Plant Physiology (1831), (German Edition, 1833, 1, p. 388), con- 

 tains a refutation of SCHRADER' s view. 



The principles of chemistry had now, however, been established on a wider 

 basis, and the answer given to the question was totally different. WIEGMAN 

 and POLSTORFF (1842), by their researches, clearly proved the correctness of 

 the view now held. 



Closer inquiry into the conditions governing absorption shows us clearly 

 that these substances pass through the external walls of plants in a state of 

 solution, since these walls are impermeable to solids. Not only the medium 

 of solution, water, but also, as a general rule, the materials of the ash are taken 

 up through the cells of the root ; only in rarer cases, e. g. in many epiphytes, 

 do the leaves take part in the absorption. The law of osmosis governs this 

 absorption and determines the nature and amount of the mineral matter 

 absorbed. 



The nature of the salts depends, in the first instance, on the permeability 

 of the protoplasm, but exhaustive researches are still required to show to what 

 degree protoplasm is permeable to the inorganic salts which in nature come 

 into relation with the plant. So far it has been clearly established that the 

 ash of plants contains by no means all the mineral matters which occur in the 

 soil or in the water. Aluminium, for instance, is widely distributed in nature, 

 and many of its compounds are soluble in water, but, nevertheless, it is absent 

 entirely, or almost entirely, from the majority of plants, although it does occur 

 abundantly in a few species. On the other hand, we know that iodine is present 

 in sea-water only in such small amounts that it can scarcely be detected, yet 

 many Algae take it up in relatively large quantities. Generally speaking, the 

 mineral matters occur in the plant in proportions quite different to what they 

 do in the outside world. This is illustrated by WOLFF'S (1871, I, 132) analysis 

 of the ash of Lemna trisulca, as compared with that of the water in which the 

 plant was grown. 



Minerals present in 100 parts of Ash. 



K 2 O Na 3 O CaO MgO Fe 2 O 3 P a O s SO 3 SiO 2 Cl 



Water . 5.15 7-60 45.56 16.00 0-94 3-42 10.79 4-23 7-99 



Lemna . 18-29 4'Q 6 ai-86 6-60 9-57 11-35 7-9 1 l6 -5 5'55 



From this summary it is impossible to say whether these substances occur 

 in the cell-wall or in the protoplasm ; if in the latter we are still ignorant as to 

 the permeability of protoplasm to these materials. Because a large amount 

 of iron is taken up it does not follow that the protoplasm is easily permeable 

 to it, and, conversely, the fact that lime is present in relatively less amount 

 in the plant than in the water must not be taken as indicating that protoplasm 

 is less permeable to that substance. We have already seen, in speaking of 

 osmosis, that, given that protoplasm is permeable to a substance, no storage 

 can take place in the plant unless the substance be altered in some way after 

 its absorption. The most obvious example is the coloration which is produced 

 by very dilute solutions of methylene blue. This is only possible if the methy- 

 lene blue becomes separated out in insoluble combinations. We are, however, 

 ignorant in individual cases wherein the changes in inorganic substances con- 

 sist which prevent their exosmosis and facilitate their accumulation. From 



