PHYSIOLOGY 



193 



It was first asserted by BEKTHOLLET (1803), and afterwards emphasised by 

 LIEBIG and C. SPRENGEL, that the mineral salts contained in plants, and once 

 supposed to be products of the vital processes of the plants themselves, were 

 essential constituents of plant food. Conclusive proof of this important fact was, 

 however, first obtained by the investigations of WIEGMANN and POLSTOHFF ( 16 ). 



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 ( 17 ). 

 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. 



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 amounts of the same substances in the straw, 

 is worthy of especial notice. 



In a similar way the marine Algae, according to WILLE ( 18 ), economise the 

 nitrates and phosphates, which exist in small proportions in sea-water, but not 

 potassium and magnesium, the supply of which is abundant. 



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. 



The Process of Absorption ( 19 ), As all matter absorbed by plants 

 must, as a rule, pass through continuous cell walls, no solid food can 

 be taken up. The only exception to this rule occurs in the amoeboid 

 developmental stages of the lower plants (amcebae and plasmodia), 

 which, as they have no cell walls, are in a condition to take up 

 and again extrude solid matter (particles of organic or inorganic 

 substances). 



The fact that plant cells are completely enclosed by continuous 

 walls renders it necessary that food, to pass into the cell, must be 

 either liquid or gaseous. In this condition the constituents of plant 

 food are, however, imperceptible, and thus the manner of plant 

 nutrition remained for a long time a mystery, and it was only during 

 the eighteenth century that the nature of the nourishment and 

 nutritive processes of plants was recognised. 



The nutrition of the plant is dependent upon the permeability of 







