DEPARTMENT REPORTS 55 



Wallace, in his work on Darwinism, says: 



"People do not see the constant and daily search after food, the failure to ob- 

 tain wliich means weakness or death; tlie constant effort to escape enemies; the 

 ever-recurring struggle against the forces of nature." 



This statement, made with reference to animals, seems to apply equally 

 well to plants. Again I quote from Wallace: 



"It is not so commonly known that if a garden is left to becora<^ altogether wild, 

 the Aveeds that first take possession of it, often covering the whole surface of the 

 ground with two or three different kinds, will themselves be supplanted by others, 

 so that in a few years many of the original flowers and of the earliest weeds may 

 alike have disappeared. This is one of the very simplest cases of the struggle for 

 existence, resulting in the successive displacement of one set of species by another; 

 but the exact causes of this displacement are by no means of such a simple nature. 

 All the plants concerned may be perfectly hardy, all may grow freely from seed, 

 yet when left alone for a number of 5'ears, each set is in turn driven out by. a suc- 

 ceeding set." 



DeCandalle says: 



"All the plants of a country are at war with each other, each one struggling to 

 occupy ground at the expense of its neighbor." 



Again from Wallace: 



"Besides this direct competition, there is one not less powerful arising from the 

 exposure of almost all plants to destruction by animals. The buds are destroyed 

 by birds, the leaves by caterpillars, the seeds by weevils; some insects bore into the 

 trunk, others burrow into the twigs and leaves; slugs devour the young seedlings 

 and the tender shoots, wireworms gnaw the roots. Herbivorous mammals devour 

 many species bodily, while some uproot and devour the biu'ied tubers. 



"Besides having I0 protect themselves against coiupeting plants and against 

 destructive animals, each species can sustain a certain amount of heat and cold, 

 each requires a certain amount of moisture at the right season, each wants a 

 proper amount of light or of direct sunshine, each needs certain elements in tbf» 

 soil. The struggle for existence in plants is threefold in character and infinite m 

 complexity. 



"We must not overlook the fact so well established that one of the greatest 

 points to be gained by migration, is to enable the tlowers of different stocks of a 

 species to be cross-fertilized and thereby improved in vigor and productiveness. 



"No doubt many of these facts are familiar to you, if so." all the better, for we 

 can then discuss their meaning to greater advantage. They teach emphatically 

 some of the good reasons for a rotation of crops, which means new fields for old 

 or for new plants. As Lubbock says in his "Flowers, Fruits and Leaves": 

 'Farmers have found by experience that it is not desirable to grow the same crops 

 in the same field year after year, because the soil becomes more or less exhausted. 

 The powers of dispersion possessed by many seeds are a great advantage to the 

 species. Moreover they are also advantageous in giving the seed a chance of 

 germinating in new localities suitable to the i-equirements of the species.' " 



People often think of animals as traveling from place to place, but for- 

 get that plants can travel as well. By an almost infinite number of de- 

 vices seeds and fruits of plants flee from the parental spot on the wings 

 of the wind, float on currents of ocean, lake and river. They are shot 

 by bursting pods and capsules in every direction. With hooks and glands 

 they cling to the coverings of animals. Allured by brilliant colors, birds 

 and many other animals seek and devour the fruits of many plants, the 



