296 PLANT-BREEDING 



Tall plants are less branched than smaller ones, and bear 

 a correspondingly smaller number of capsules and of seeds. 

 Potatoes have often been an object for comparative measure- 

 ments of their different parts and a survey of their correlations 

 shows that these are quite numerous. It may suffice to 

 merely point out the parallelism between the average number 

 and weight of the tubers on a plant, the number of stems, 

 and the amount of starch in the potatoes. 



Many other instances could be given. They all give 

 proof of a high degree of parallelism between the different 

 organs of a plant, increasing development of one quality 

 being combined with increase or decrease of other points 

 according to definite rules. By this means selection may 

 be guided, as we have seen in a previous chapter. But the 

 cultural treatment may be furthered also by the observation 

 of such correlations, because in stimulating the develop- 

 ment of some parts we may expect to increase the size or 

 weight of others at the same time. An exact knowledge 

 of these phenomena is thus of paramount interest for agri- 

 cultural practice, and the difficult and tedious study of all 

 its features should be diligently carried through until definite 

 principles for the practical work may be derived from it. 



Inquiring into the causes of the parallelism of apparently 

 independent characters, we have first to state that it is often 

 difficult to decide whether the observed combinations are 

 due to internal or to external causes. Among the facts 

 I have quoted, there can be no doubt that at least a large 

 majority are due to external influences. On the other hand, 

 more intimate or internal* relations may be hidden in some 

 instances without our being able to discern them and thus 

 to classify our group of phenomena. Our only way is to 

 leave them out of consideration until further investigation 

 will have pointed them out and to consider our facts as 

 uniform and as the result of the same causes. 



