SECT. II 



PHYSIOLOGY 



233 



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 miist 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 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. 



The actual proportions of the more important ash constituents of some well- 

 known plants can be seen from the following table of ash analysis by E. Wolff. 

 The table also shows exactly what demands those plants make upon the soil, that 

 is, what substance they take away from it, in addition to the nitrates which do 

 not appear in the ash. 



Plants. 



Rye (grain) . . 

 Rye fstraw) . . 

 Pea (seeds) . . 

 Pea (straw) . . 

 Potato (tubers) . 

 Grape (fniit). . 

 Tol)aceo (leaves) 

 Cotton (fibres) . 

 Spruce (wood) . 



Ash in 



100 parts of 



dry solid 



matter. 



2-09 

 4 •46 

 2-73 

 5-13 

 3-79 

 5-19 

 17-16 

 1-14 

 0-21 



100 Parts of Ash contain 



KoO 



32-10 

 22-56 

 48-10 

 22-90 

 60-06 

 56-20 

 29-09 

 .36-96 

 19-66 



NaoO 



I 



1-47 

 1-74 

 0-9S 

 4-07 

 2-96 

 1-42 

 3-21 

 13-16 

 1-37 



CaO 



2-94 



8-20 



4-81 



36-82 



2-64 

 10-77 

 36-02 

 17-52 

 3397 



MgO 



11-22 

 3-10 



7-99 

 8-04 

 4-93 

 4-21 

 7-36 

 5-36 

 11-27 



SO, 



1-28 

 4-25 

 3-42 

 6-26 

 6-52 

 5-62 

 6-07 

 5-94 

 2-64 



SiO.2 



1-37 

 49-27 



0-91 

 6-83 

 2-04 

 2-75 

 5-77 

 2-40 

 2-73 



CI 



0-48 

 2-18 

 1-59 

 5-64 

 3-46 

 1-52 

 6-71 

 7-60 

 0-07 



The great difference brought out by the table in the proportions of the more 

 important phosphoric acid and of the less essential silica and lime contained in Rye 

 and Pea seeds, as compared with the amoiuits of the same substances in the straw, 

 is worthy of special notice. 



In the preceding table the figures do not express absolutely constant proportions, 

 as the percentage of the constituents of the ash of plants varies according to the 

 character of the soil ; thus, the proportion of potassium in Clover varies from 9 to 

 50 per cent ; the proportion of calcium in Oats from 4 to 38 per cent. 



D. Assimilation of 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 proto- 

 plasm and cell-wall. Such water can be promptly given off by evapora- 



