LEAKE AND KAM I'llASAD 



Having ascertained the types it is next necessary, ])v a com- 

 parative study of the points or characters, by which these diifer, 

 to determine the main characteristics. The unit of tliis series of 

 observations is now the character. But the investigation is not 

 hniited to the determination of these alone ; by crossing types 

 which differ in certain definite characters and tiacing the be- 

 haviour of these in the offspring, it is possible, not oidy to ascer- 

 tain with some clearness what constitutes a unit character but, 

 also, to obtain information as to the behaviour of these units under 

 definite conditions — information which permits efforts aimed at 

 improvement to be undertaken with some confidence. 



The plant may be imagined to consist of an aggregate of unit 

 chai-acters, — the presence or absence of an ultimate common base 

 is not here a matter for concern c./. de Vries (11) ceitain combi- 

 nations of which are to be found in nature. Improvement consists 

 in a rearrangement of these units with the pioduction of a plaut 

 more suited to the conditions of growth and having a more valu- 

 able produce. Regarded in this light the possibility of effecting 

 any improvement is seen to depend very largely on the clearness 

 with which the objective receives definition in the nnnd. In this 

 way only can the most suitable method of obtaining that combi- 

 nation, and the most suitable types to use as parents, be selected 

 with any degree of certainty. 



Although the plant forms the central object of study it is not 

 possible to limit observation to this one aspect. The inter-action 

 between the plant and its surroundings is intimate. It is further 

 necessary, therefore, to determine the natui'c of the surroundings 

 or environment and also, if possible, the conditions which render 

 one type more suitable than another for growth under a particular 

 set of conditions. In the broadest sense the environment includes 

 not only the physical conditions by which the plant is surrounded 

 but also the organic. In other words, there exists an inter-action, 

 not only between the plant and the soil, climate and such physical 

 conditions of growth but also between the plant and its neighbours 

 — in the case of a field crop usually plants of the same species — 

 insects, harmful or otherwise, and organisms causing disease, 



