The Struggle for Existence. 223 



duce grown under the same conditions as to manuring might 

 be composed very differently in two different seasons." 



The influence due to the special medium through which a 

 particular plant-food, such as nitrogen, is presented to the 

 plant, is aptly illustrated in the following words : " Because 

 a particular grass, or other plant, is little benefited by 

 ammonia-salts for instance, it does not follow that it will not 

 be favoured by nitrates ; nor, because if while growing in 

 association with other species it may not be specially benefited 

 by a particular manure, does it follow that it would not 

 derive advantage from the same substance when growing 

 separately." 



Nearly all the plants on the plots are perennials, very few 

 are annuals, Bromis mollis being the only case amongst the 

 grasses. The advantage possessed by deep-rooting over 

 surface-rooting plants was well brought out in the droughty 

 season of 1870, when the latter suffered considerably from 

 lack of moisture. The locomotive power of underground 

 stems is of great use to some plants ; " the stock continues to 

 grow at one end, year after year, the opposite end gradually 

 dying away. In the course of a few years the plant therefore 

 occupies quite a different position from that which it a,t first 

 had." Notwithstanding the general rule that the chief effect 

 of nitrogenous manures is to favour the extension of foliage 

 and give it depth of colour, whilst that of mineral manures 

 is to encourage stem formation and the production of seed, 

 and notwithstanding that excessive nitrogenous manuring 

 prolongs the development of the vegetative organs till 

 perhaps the resources of the plant are exhausted or the 

 season is over, whilst excess of mineral manures may 

 induce premature ripening, yet so far as the experiments 

 have gone no absolute change in the distinctive form of any 

 plant has been effected by the prolonged use of the different 

 manures, though changes of degree are sometimes very 

 marked, as in the tufts of Dactylis glomerata. 



The battle for life between the various species of plants 



