176 



BOTANY 



It was first asserted by C. Sprengel, and afterwards emphasised 

 by LiEBlG, that the mineral salts contained in plants, and once sup- 

 posed 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 



WlEGMANN and POLSTORFF. 



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 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. 



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. 



Plants which require a large amount of potassium, such as the Potato, Grape- 

 vine, and Coffee-tree, are termed potash plants. 



In the preceding table the figures (do not express absolutely constant propor- 

 tions, 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. — As all matter taken up by plants 

 must, as a rule, pass through continuous cell walls, it must be absorbed 

 in a liquid or gaseous state. The only exception to this rule occurs in 

 the amoeboid forms of the lower plants {Amoebae and Plasmodia), 

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

 again extrude solid matter (small animals, living or dead, also plants 

 and particles of 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 



