466 RELATION TO ENVIRONMENT. 



physiological optimum as regards the necessary amount of water 

 in the soil, or in all the environment, including moisture in the 

 air. For many plants with which we are familiar the physio- 

 logical optimum of water in the soil is when the soil is percep- 

 tibly moist, not dry to the feel, nor so wet as to be saturated, or 

 so that water can be squeezed from it by pressure in the hand. 

 These plants, which usually have thin .leaves for rapid trans- 

 piration, if the water increases to the point of saturation in the 

 soil, become sickly or die, because there is not sufficient air in 

 the soil to enable the roots to absorb enough water. On the 

 other hand, if the moisture in the soil diminishes, as during a 

 dry period, the plant often wilts and even dies. 



903. Very dry soil. In regions where the soil is very dry for 

 most of the year, the plants accustomed to an abundance cf 

 moisture will not grow. But here we find plants growing which 

 through long time became accustomed to the conditions by being 

 modified, i.e., the root system is enlarged, so there is more sur- 

 face for absorption, while the leaf and stem system is reduced, 

 so the leaves are small and thick, thus reducing the surface for 

 transpiration, while at the same time providing a water storage 

 in the thick leaves, succulent leaves, or tuberous roots. 



904. Excess of soil-water. Now if we turn to the situations 

 where the soil is constantly saturated with water, or the ground 

 is covered with shallow water, where plants accustomed to moist 

 soil could not grow, we likewise find plants growing which have 

 become modified so as to fit them to succeed under these condi- 

 tions. The system of roots and root-hairs is reduced because of 

 the water and unfavorable conditions for absorption, and cor- 

 respondingly we find the leaf system usually reduced so that 

 transpiration is lessened. In this way water in medium or 

 scarce amounts, or in excess, exercises an influence on the plants. 

 But the character of the plants cannot be determined alone by 

 the amount of water in the soil. The humidity of the air, the 

 drying effect of wind, the effect of heat and light, as well as the 

 chemical and physical properties of the soil, are to be taken into 

 account 



