dactylon) requires more than 1232 degree-hours (summation of 

 degrees above 0°F per hour per day) and day temperatures 

 above 50°F to show measurable growth. If the night temperature 

 falls below 50°, the day temperature must be correspondingly 

 higher to raise the number of degree-hours to the essential 



minimum.^ ^^ 



Aquatic plants (hydrophytes) such as the water-lily [Nym- 

 phaea) and the submerged eel-grass (^ostera) require a continuous 

 supply of water; in contrast are numerous desert plants such as 

 the creosote bush (Larrea divaricata), prickly pear (Opuntia), and 

 others (xerophytes) which are able to endure long periods with- 

 out rain. Many annuals in the Arizona desert flourish during the 

 cool weeks of late winter and early spring when the rainfall has 

 been adequate. In the Great Plains, western wheatgrass (Agro- 

 pyron smithii) requires more moisture and less heat for good growth 

 than buffalograss {Buchloe dactyloides), and by the same token, for 

 the germination of seeds and later growth of many species the 

 soil moisture must be adequate at the periods of the year when 

 the temperatures are suitable. Some grasses, notably the sixweeks 

 grasses {Sporobolus microspermus, Festuca octqflora, Bouteloua barbata, 

 B. parry i, Aristida adscensionis), can grow and mature during the 

 summer in southern Arizona when soil moisture is available for 

 only a short time. Many kinds of weeds grow in size in almost 

 direct relation to the supply of available soil moisture, Russian 

 thistle {Salsola kali), for example, growing from only an inch or 

 two to as much as two feet with a spread of as many feet or more, 

 in response to different amounts of soil moisture. In addition, an 

 insufficient depth of soil may prevent development of the root 

 system of some deep-rooted grasses such as orchardgrass {Dactylis 

 glomerata), leaving the area open to invasion by shallow-rooted 

 grasses or weedy plants. 



An adequate supply of mineral nutrients in the soil, including 

 the trace elements, is most important for the growth of plants. 

 This phase of physiology is very inadequately understood for wild 

 plants, but the decline in vigor of some species that apparently 

 have high nutritional requirements, especially in grasslands suf- 

 fering from heavy grazing or erosion, permitting the invasion of 



Ecological CHaracteristics of Species &. Popvmlatioxms • 11 



