148 THE INDIVIDUAL ORGANISM 



of one or more of these trace elements in particular soils, and the ailments 

 so caused have come to be known as mineral deficiency diseases. They bear 

 a certain analogy to vitamin deficiencies in animals. Mineral deficiencies 

 are much more prevalent in some regions than in others, and are deter- 

 mined largely by the mode of origin and parent rock materials of the soils. 

 In the Atlantic coastal plain the soils have been largely derived from 

 materials carried to ancient seas by ancient streams. The sorting of the 

 sediments by rivers and waves, and the leaching to which the derived 

 soils have been subjected have left many soils in this region deficient 

 in certain elements — especially manganese, zinc, and copper. The de- 

 ficiencies are especially marked in Florida, which lies farthest from the 

 Piedmont and Appalachian sources of these materials, and which has 

 unusually heavy rainfall to cause leaching. The various roles played by 

 the trace elements in plant physiology are still poorly understood. 



Other elements, such as sodium, chlorine, silicon, aluminium, nickel 

 and perhaps others, are often beneficial to the growth of plants, and some 

 of these may be found necessary to particular species of plants. 



Intake and transportation by roots. The root hairs penetrate into 

 the crevices of the soil, pressing close against the surfaces of the soil 

 particles. These particles are wet with soil moisture — a dilute solution of 

 salts. Water and solutes pass through the permeable walls and the semi- 

 permeable cell membranes into the epidermal cells of the root tip, thence 

 across the cortex into the stele, and along the conductive tissues to the 

 rest of the plant. Some of the factors involved in this movement are well 

 known, but others cannot yet be explained. Our lack of knowledge con- 

 cerning so familiar a phenomenon as absorption by roots is another 

 reminder of how close the frontiers of science lie, and how much remains 

 lo be learned about even the simpler life processes. 



Absorption of Water. We may regard the epidermal cells and root 

 lairs as being immersed in the dilute soil water solution, and separated 

 <rom it only by the living cell membrane (the cell wall being freely per- 

 meable). The cell membrane is said to be semipermeable, for while it is 

 permeable to water and such solutes as are ionized or have small mole- 

 cules, it is increasingly impermeable to solutes with increase in the size 

 of their molecules. It is also differentially permeable, as we shall see. 

 Proteins, fats, and large carbohydrate molecules pass through the cell 

 membrane with great difficulty, if at all. 



It is a well-known fact that water moves through a semipermeable 

 membrane from the region of lesser to that of greater concentration of a 

 solute or solutes. This phenomenon is called osmosis. The concentration 

 of solutes in the epidermal cells of the root tip is normally greater than 

 in the soil solution, and water therefore enters the cells. Osmosis accounts 

 for much of the absorption of water by roots, but it is not the whole 



