Plants with seeds varying in period of dormancy have advan- 

 tages in surviving unfavorable conditions. Retention of viability 

 for several years appears advantageous to plants such as Minuar- 

 tia stricta which have little competitive capacity,!^^ to many 

 legumes, and others. Some seeds of annuals lie dormant in the 

 soil in grasslands for a number of years before bare areas appear. 

 Seedlings in such spots are likely to survive but those that ap- 

 pear among other plants usually die. Annual species are often 

 dependent for dispersal and survival upon large supplies of seeds 

 and high rates of germination; and low production may restrict 

 the distribution of a species, as in northwestern Montana where 

 one of the factors in maintaining a stable ecotone of aspen 

 woodland and grassland has been stated to be the poor seed 

 crops of the former, caused probably by unfavorable climatic 

 conditions. 138 The restricted distribution in the Sahara and Negev 

 of the shrub Calligonum comosum to coarse sandy soils where the 

 rainfall is low, is probably because this substratum provides con- 

 ditions needed for germination, as it is known that the seeds fail 

 to germinate in the light, at high temperatures, or even in close 

 contact with water. ^^s 



Figure 1-18. Aster foliaceous is capable of invading eroded 

 depressions by means of rhizomes, but seedlings would have 

 difficulty. A few plants of Geranium, Eriogonum, Achillea, 

 and Vicia are intermixed here. Steel tape shows former soil 

 level. August, 10,000 ft, Utah. (U. S. Forest Service.) 



