INFLUENCE OF ENVIRONMENT ON PLANT LIFE. 103 



portion of the year the environmental conditions surpass the hmits for 

 both growth and hfe in the plant itself; the conditions in their adverse 

 period do not surpass the limits for the retention of life in the seed. 

 During a comparatively short period the environment may allow 

 germination, growth, and the production of more seeds, and this 

 short growing-season, together with the bridging of the adverse period 

 by dormant seeds, constitute the conditions that are necessary and 

 essential in order that such a plant may maintain a permanent place 

 in the vegetation of its region. 



External conditions frequently surpass even the life-limits of many 

 seeds; in such cases it must, of course, follow that the plant can not 

 become a permanent part of the natural vegetation. This is probably 

 true of the majority of cultivated plants that do not volunteer in the 

 second season. A tropical plant, such as the castor-bean, may make a 

 luxuriant growth in the temperate summer, but its seeds must be pro- 

 tected through the winter and sown each spring. 



In general, the natural plants of the temperate and frigid regions 

 must necessarily experience a longer or shorter period, usually each 

 year, with proper conditions for growth and reproduction, and they 

 exist through the adverse periods in some dormant phase. Not only 

 must the conditions of the active period lie within the limits for growth 

 and reproduction, but the period itself must be of adequate length, 

 otherwise the necessary amount of growth could not take place, and 

 fruit would not be matured. The principle that a plant, to become 

 a part of the permanent vegetation of a locality, must have an adequate 

 growing-season and must not meet its death during the remainder of 

 the year, must be regarded as fundamental to the study of all problems 

 deahng with the study of plant behavior under natural conditions. 

 This principle is commonly accepted, though perhaps seldom formu- 

 lated. 



With regard to any geographical area or region, we may conceive 

 that all plants may succeed therein, for which the physiological limits 

 for Hfe and those for growth and reproduction do not approach each 

 other more closely than do the extremes of the physical conditions in 

 the respective seasons for the given region. This view has led to a 

 form of analogy which may be termed the sieve conception of environ- 

 ments. According to this, we may regard the physical conditions of 

 the surroundings as resembling a sieve or screen, with meshes of a 

 certain magnitude, through which, as we may imagine, will pass only 

 those successful forms which withstand the most adverse conditions 

 of the environment. The analogy is but roughly applicable; to make 

 it more so we may suppose that the size of the meshes in our screen 

 is continually changing throughout the year, while the size of the 

 imaginary particles which are to be screened are also undergoing 

 continuous change with the advance of the organism from phase to 

 phase of its development. With the progress of the season the con- 



