378 PLANT PHYSIOLOGY 



although in many cases not so successful from the point of view 

 of numbers, has the great advantage of introducing new qualities 

 in the form of new combinations of characters. 



The advantage of cross-fertilization, which was first emphasized 

 by Darwin, has the same reasons at its base. According to Dar- 

 win, nature " abhors self-fertilization" as she does a vacuum, 

 and the many devices of plants to bring about cross-fertilization 

 and to inhibit self-pollination and fertilization support this view. 

 Among these devices are dicecism (see above), the ripening of 

 stigmas and pollen at different times, etc. The complex relation- 

 ships between insects and flowers, which result in cross-pollination, 

 seem to have had a decided survival value. While all these ex- 

 amples show the " abhorrence of self-fertilization," some plants 

 like Viola, Poly gala, and Alisma possess flowers which never open 

 and so must be self-pollinated. Such cleistogamous flowers in- 

 sure pollination in case the cross-pollination by the opened flower 

 fails. East and Jones (1919) have shown that inbreeding or close 

 fertilization is quite all right and not to be deprecated in case the 

 stock is a good one. In case it is bad, then inbreeding will result 

 in the continuation and intensification of bad qualities; but if the 

 organisms have no bad qualities which are dominant in crosses, 

 then there is nothing to fear. In the development of fine herds of 

 cattle, horses, and other animals, inbreeding is a very common 

 practice with no visibly bad results. Since the physiology and 

 morphology of sexual reproduction has been taken over by the 

 science of genetics based upon the discoveries of Gregor Mendel, 

 little more will be said upon this subject here. 



Determination of Sex. — The cause of maleness and female- 

 ness in organisms has always been a question of much popular as 

 well as scientific interest, but only within this century has any 

 progress been made in solving this attractive problem. In ani- 

 mals it has been observed that the males and females generally 

 differ in their chromosomal constitution. It commonly happens 

 that the male has an odd number of chromosomes in the cells of 

 the body while the female has an even number. This body (so- 

 matic) number is twice that of the gametes so that when reduc- 

 tion occurs, at the time of the formation of the sex cells, the 

 female gametes will be all alike with half the somatic number; 

 while the male gametes are of two kinds, because an odd number 

 of units cannot be divided into two equal halves. If the male 



