It has been found out by using monocular compound microscopes that concentration of dissolved substances such as sugar and other organic compounds in an epidermal cell is normally higher than the concentration of dissolved substances in the soil water. With the osmotic concentration higher inside the cell than outside, a simple osmotic system is established and water can move across the membrane into the cell. This osmotic process can be seen using monocular compound microscopes. Monocular compound microscopes are microscopes having only one eyepiece.

Once water has entered an epidermal cell, it dilutes the contents of that cell. The concentration of dissolved substances becomes lower in it than in the adjacent cell of the cortex, and water can move from it to the cortex cell by osmosis. But now a new concentration gradient is established, as water moving into the outermost cell of the cortex dilutes the contents of that cell and lowers its osmotic concentration to a point below that of the next cell of the cortex. As a result, water moves from the first cortex cell to the second cortex cell, following the concentration gradient. Again, dilution of the recipient cell occurs, a new gradient is established, and water moves on to the next cell. In this way, water can move fairly easily from the capillary films of the soil into the epidermis and thence across the cortex to the stele. The result of this movement can be seen when the cell is viewed under monocular compound microscopes. Once inside the xylem of the vascular cylinder, the water can rise to other parts of the plant body. Removal of water from the center of the root via the xylem maintains the concentration gradient from the epi¬dermis to the xylem and allows the process of water absorption to continue.

The cytoplasm of adjacent plant cells, when viewed under monocular compound microscopes, is inter¬connected by plasmodesmata to form a continuous system called the symplast. It is likely that the water can also move from an epidermal cell to a cell in the cortex, and from one cortex cell to the next, through the symplast. Once water or some other substance has entered an epidermal cell by osmosis, it can thus move to other cells through the plasmodesmata, without having to cross any addi¬tional cell membranes.

Analogous to the symplast is the apaplast, is a network composed of the cell walls and intercellular spaces. The apoplast provides another means, besides osmosis, by which water can enter the root. The cellu¬lose in plant cell walls is hydrophilic; it has a strong attraction for water molecules, and the water can flow along the cell walls from cell to cell in the apoplast across the epidermis and the entire cortex of a root without ever actually penetrating a membrane or entering a cell. Indeed, recent experiments suggest that in the normal up¬take of water by roots, movement of water in the apoplast is much more important than movement in the symplast. But the water cannot flow across the endodermis in this manner, because as seen in monocular compound microscopes, the Casparian strip acts as a barrier; it interrupts the apoplast. Consequently all water entering the vascular cylinder must cross through the living cells of the endodermis. This gives the plant an opportunity to control the movement into the stele of substances dissolved in the water.

Whether inorganic or organic fertilizer is applied to a crop, the plants primarily as inorganic ions absorb the minerals. How¬ever, the two kinds of fertilizers, which have very different physical and chemical effects on the soil as seen under monocular compound microscopes, may influence plant growth dif¬ferently in other ways; for example, organic material may possibly provide vitamin like growth stimulants, but at times it may also con¬tain substances toxic to plants.

The ions available to plants for absorption are in solution in the soil water, their concentration varying according to the fertility and the acidity of the soil and other factors. In other words, the concentration of various ions in soil water is often determined by the pH of the soil itself. When the soil minerals are not in solution, but are bound by ionic bonds to soil particles, they are not available to plants. Agricul¬tural soil management often involves changing the soil acidity to free more such bound minerals for absorption by roots. For example, the addition of lime to very acid soil in order to raise the pH may increase the availability of phosphorous, potassium, and molybdenum, but an excess of lime may decrease the availability of iron, copper, manga¬nese, and zinc. Obviously, a careful balance, appropriate to the particular crop to be grown, must be achieved for maximum yield.

The rate of absorption of each mineral by roots is essentially inde¬pendent of the rates of absorption of water and of other minerals. Each nutrient moves into the root at a rate determined by such factors as its concentration both inside and outside the root, the ease with which it can passively penetrate cell membranes, and the extent to which carrier molecules are involved. Although some of the inward movement of minerals, like that of water, is a result of passive diffu¬sion along a concentration gradient, the rate of absorption is often greater than would be possible by passive diffusion alone-which means that facilitated diffusion is taking place. Moreover, plants can often take in a mineral that is in higher concentration inside the root cells thin in the soil solution. Active transport is clearly involved. The plant expends energy in the process of procuring the mineral nutrients essential to its continued existence. As will be evident throughout this book, active transport is the rule rather than the exception in most kinds of organisms, whether plant or animal, when substances are moving across the membranes of living cells.