CHIEF ENVIRONMENTAL CONDITIONS. 145 



The direct mechanical effect of flowing water is familiar to everyone, 

 and it must be accounted of great importance in determining the 

 nature of the vegetation of many stream margins, as well as of streams 

 themselves, of intermittently flooded stream-channels in the arid 

 regions, and of sea and lake beaches. Other factors undoubtedly are 

 coeffective in such cases, however. 



The influence of animals upon vegetation has not been much em- 

 phasized in plant ecology, but it is undoubtedly of considerable impor- 

 tance in many cases, as when seeds are thus mechanically destroyed in 

 such large numbers that the establishment or spread of a species is 

 rendered practically impossible. This factor in distribution is usually 

 operative only on certain developmental phases of the plant ; often the 

 seedling stage is preeminently in danger of destruction by animals. In 

 agriculture and horticulture — practical ecology under more or less 

 artificial conditions — the influence of animals is of prime importance 

 and has perforce received much attention. Fences, traps, scarecrows, 

 insecticides, and even trespass warnings are material evidences of the 

 importance ascribed to the direct mechanical influence of animals upon 

 the plant population. For the most part, methods have been readily 

 devised for more or less thoroughly removing this source of danger to 

 cultivated plants . Among the different groups of animals, insects have 

 probably been the least easily combated, and much attention is still 

 being devoted to this important destructive factor. It will be an 

 interesting and important chapter of agricultural ecology when the 

 geographical distribution of various forms of animals is correlated with 

 that of the regions where the various crops may be successfully grown. 

 Livingston^ has mentioned the apparent importance of animals in de- 

 termining the very existence of irrigated seedlings in the dry season at 

 Tucson, and we have often made observations in that same region that 

 suggest a rather important relation between certain plant-fornis and 

 animal activity. It seems probable that the destructive action of 

 animals is relatively more important in arid regions than in most others. 



3. FAVORABLE INFLUENCES OF MECHANICAL CONDITIONS. 



Wind, water, animals, etc., as is well known, frequently accelerate 

 the spread of plants throughout large areas. In the majority of these 

 cases it is a dormant phase, as of seeds, that is moved about. It is 

 also true, however, that fragments of the plant-body other than seeds 

 may be torn or broken away, being removed to another locality and 

 there continuing development. Such is often the case with willow 

 twigs, which float downstream and find lodgment and conditions for 

 growth in a muddy bank. The movement of cactus branches in the 

 arid regions of the American Southwest has been mentioned above. 



1 Livingston (19066), page 58. 



