302 ELEMENTARY STUDIES IN BOTANY 



subject to disease. In certain kinds of crops, these symp- 

 toms suggest some kind of protection against excessive cold 

 or heat. 



10. Water. Water may be regarded not only as a 

 necessary condition for plant work, but also as a material 

 used in the manufacture of plant food. The active plant 

 body is really saturated with water, and the living substance 

 (protoplasm) cannot work effectively unless it contains an 

 abundance of water. In addition to the water that puts a 

 plant in working condition, a much smaller amount is used 

 in the manufacture of carbohydrates (sugar and starch) by 

 leaves. A saturated plant exposed to air means a continual 

 evaporation from its free surfaces, and notably from its 

 leaves. This loss must be made good by the continuous 

 entrance of the water of the soil into the roots, so that there 

 is a continual movement of water through a plant. When the 

 loss of water exceeds the supply, the immediate effect on the 

 plant is seen in its " wilting." It is this condition that all 

 plant cultivation must guard against. 



In the case of many field crops, there is no control of the 

 water supply, and the farmer must take his chances that the 

 rainfall will be adequate. But in gardens it is usually possible 

 to supplement a failing water supply, and in dry regions where 

 irrigation is used the water supply is under control. In all 

 cases where water is supplied in some way by the cultivator, 

 it is necessary to learn the needs of the crop. Generally too 

 much water is supplied, which injures both the plant and the 

 soil. It should be recognized also that the amount of water 

 required varies with the period of the plant; for example, 

 more water is needed during the growth of the plant than 

 during either seed-germination or ripening. The problem 

 is not merely to induce plants to grow by just keeping them 

 from wilting, but to enable them to grow vigorously. Too 

 little water during the growing period results in smaller 

 leaves and less work; while a supply of water that favors 



