THE ENVIRONMENT 



309 



growing season before entering dormancy. For ex- 

 ample, at the base of most twigs resistant buds are 

 formed. These buds contain embryo leaves, stored 

 food, and cells capable of rapid growth, all enclosed 

 in a protective envelope of bud scales, modified leaves. 



PLANT-WATER RELATIONS 



The structures of plants may reflect their water 

 relations in many ways; however, most if not all of 

 these reflections also might be adaptations to tem- 

 perature and other environmental factors. In habi- 

 tats having contrasting dry and wet seasons the 

 perennial plants often are deciduous; the deciduous 

 period, that of leaf loss and general dormancy, is 

 in the dry season. In contrast, the wet season is 

 one of leaf development, growth, reproduction, and 

 general functional activities of the plant. 



The associations between plants and available 

 moisture are so important that many types of plants 

 are recognized on this basis. The three primary types 

 are hydrophytes (water plants), mesophytes (moist 

 plants) and xerophyks (dry plants) (Figure 17.5). 

 Hydrophytes have very high to maximum available 

 water, their substrate is either water or is covered 

 by water, and they usually are found in moist 

 habitats. The primary characteristic of these plants 

 is their ability to survive under the minimal oxygen 

 conditions of water. Structurally, the main macro- 

 scopic modifications are smaller root systems (if roots 

 are present) and the tendency for underwater vegeta- 

 tive structures to differ from surface or above-water 

 structures. 



There are five subtypes of hydrophytes. Floating 

 hydrophytes, such as duckweed (Lemna) and mosquito 

 fern (Azola), contact only air and water. Suspended 



Figure 17.5 Plant-woter relationships as indicated by hydrophytes, mesophytes, and xerophytes. 

 Hydrophytes: 1. suspended; 2. flooting; 3. submerged; 4. floating-leaved onchored; 5. emergent 

 anchored. Xerophytes: 6. ephemerol onnual; 7. succulent perenniol; 8. nonsucculent perennial. 

 Mesophytes: 9. sun plant; 10, shade plants. 



