DISPERSION AND GERMINATION OF SEEDS. 223 



growth, or upon a soil which is already pre-occupied by other 

 plants. All the plants of a given district may be regarded as 

 ^t war with each other. The arborescent species prevent, by 

 the extent of soil which they occupy, the vegetation of species 

 of a humbler growth. Each has to struggle into existence 

 against a host of competitors, for nature, although she has 

 been prolific of the seeds of life, has limited the supply of 

 room and food. A number of ferns, for example, which may 

 be growing on a hill-side, will, by their pre-occupation of the 

 soil, successfully maintain their ground against all other 

 intruders for ages, notwithstanding the facilities afforded to 

 other plants for the dispersion of their seeds. If any chance 

 seed should be borne to this spot by any of the agencies which 

 we have enumerated, or by other causes, it cannot germinate 

 among them, as they absorb all the food from the soil. 



The seeds which have been thus unfavorably located, retain 

 their vitality for a longer or shorter period of time. Such as 

 have very thin and delicate integuments, will lose their 

 germinating power after a few weeks' exposure ; so also oleagi- 

 nous seeds will in general, decay much sooner than such as 

 contain albumen. Other seeds, on the contrary, will retain 

 their vitality for an indefinite period of time. This is the case 

 with the Leguminous plants, the seeds of which piay be kept 

 for years without any material detriment to their germinating 

 power. Peas taken from the herbarium of Tournefort, where 

 they had remained for more than one hundred years, were 

 made to germinate in the botanical gardens at Paris. Those 

 changes by which the ovule is changed into the mature seed, 

 appear to be all made with a special reference to any mishaps 

 which may befall it when thrown on the charity and care of 

 nature by the parent plant, as well as to provide it with a 



