THE BIOLOGY OF 

 FLOWERING PLANTS 



CHAPTER I 



THE ABSORPTION OF WATER AND SALTS 



§ I. The Necessity of Water for the Plant. § 2. The Origin and 

 Nature of Soil. § 3. The Root System. § 4, The Absorption of 

 Water. § 5. The Absorption of Salts from the Soil. § 6. Excep- 

 tional Means of absorbing Water and Salts. 



§ I. The Necessity of Water for the Plant 



The necessity of water for the plant is fourfold, 

 (i) Abundant water is essential to the active life of the 

 protoplasm. The immense and varied chemical activity, 

 the metabolism, of the living substance proceeds only when 

 the colloids of the protoplasm are saturated with water. 

 The resting seed, with metabolism reduced to a minimum, 

 is characteristically dry ; germination and renewed activity 

 set in after the absorption of an amount of water frequently 

 greater than the total weight of the dry seed. (2) In 

 particular a sufficient supply of water is necessary to the 

 most obvious part of the growth process — extension. The 

 embryonic plant cell is filled with protoplasm. The great 

 increase in size which takes place as the embryonic cells 

 pass into one or other of the types of adult tissue, is linked 

 to the formation of vacuoles containing a watery solution — 

 the cell sap. The increase in volume of the vacuole is 

 due to absorption of water through osmotic pressure ; to 

 the development of the turgor pressure of the sap is due 

 the extension of the elastic cell wall. The cell sap of 



I B 



