Too Little Water
Most plants do not exhibit dramatic responses to too little water. Instead, growth slows. The foliage may wilt, and brown leaf tips may develop. Lower leaves turn yellow and fall off. The surface of the growing medium shrinks away from the growing container, leading to the deterioration of the fine root system. While most plants prefer a short dry period in between waterings, some require even moisture in the root zone to survive. Poinsettias, for example, need this to maintain their green leaves, brightly colored bracts, and cyathia (true flowers). Color may be lost and leaves may fall if this requirement is not met.
Too Much Water
As a rule, plants that receive too much water cease forming new leaves. The most recently formed leaves turn dark green, with the margins turning tan and cracking. The lower leaves wilt; in time, they drop or dry slowly on the plant. The stems become dark and mushy; eventually they rot. The surface of the growing medium becomes green with algae. All sorts of mosses and ferns may eventually develop on the surfaces of the growing medium and container. Watering needs to be monitored closely, as some plants need less moisture than others. Such is the case with established Sansevieria, which will rot if watered as much as other plants.
Too Little Humidity
Fortunately, most green plants thrive even in relatively low humidity, if watered properly and regularly. When grown with too little humidity, most plants cease forming leaves. The youngest leaves become yellow, smaller than normal, and crinkly at the edges. The stems become wiry as the whole plant shrinks in size. The oldest leaves may dry and drop permanently. Thin-leaved plants, such as the Boston fern, exhibit severe leaf shrinkage when they receive too little humidity.
Too Much Humidity
In general, most plants develop new leaves that are covered with yellow and tan spots. In time, the water-soaked lesions grow larger, and the centers of the spots rot. In many cases, roots begin to develop on above-ground stems. Few if any developing flower buds mature into functioning flowers. The oldest foliage on the plant may lose its color, collapse, and decompose.