86 Cook — Hybrids and Mutations. 



PHYSIOLOGICAL EXPLANATION OF VIGOR OF HYBRIDS. 



A physiological explanation of the vigor of sterile hybrids 

 has been sought by supposing that the bodily energy whicli in 

 other plants or animals goes into reproductive parts and pro- 

 cesses here gives a reinforcement of growth, as often occurs after 

 castration. This idea might find some ai)plication with the 

 adult organism, but the unusual vigor is often apparent far in 

 advance of the reproductive stage, and even in very young indi- 

 viduals. A nursery of the coffee mutation called " Maragogii)e " 

 affords a striking contrast by the side of one planted with the 

 parent "Arabian" type, and a similar precocity of vegetative 

 vigor is found in many hybrids. The diminution of repro- 

 ductive efficiency is not, evidently, the only difference, and 

 further facts must be taken into consideration if we are to gain 

 a suggestion of how the body of an organism may gain in vigor 

 after the power of perpetuating the type has declined. 



THE STIMULATION OF GROWTH BY CROSSING. 



Tht 



le general antithesis between growth and reproduction does 

 not suffice to explain the vigor of sterile hybrids, but by con- 

 sidering the cytological i)hase of these i)rocesses a somewhat 

 more promising clue may be found. 



Growth consists, among the higher plants and animals, of a 

 long series of cell divisions, while reproduction requires, on the 

 contrary, a conjugation or union of cells. It has long been 

 su))posed that the chief result of fertilization is to stimulate the 

 cell divisions upon which the growth of the new individual de- 

 ])ends,and that inbreeding produces defective organisms, because 

 this stimulation is inadequate. Darwin says, for example, that 

 '' crossing, by itself, does no good " unless the individuals crossed 

 differ somewhat in characteristics or conditions of growth. 

 Crosses between organisms of a moderate degree of diversity are 

 more vigorous and more fertile than if either of the parent stocks 

 is inbred, but it appears that the limit of fertility is reached 

 much sooner than that of vegetative vigor. This fact corre- 

 sponds with what has been learned from the microscopical 

 study of cells — that the processes of growth or cell division are 

 much simpler than those involved in reproduction b3Mneans of 

 the conjugation of cells. It might be supposed, therefore, that 



